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Improving Corrosion Resistance of Stainless Steel

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Introduction

            Industrial practices and experience show limited ways of improving the corrosion resistence of stainless steels.

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  Corrosion resistance can be improved by altering the steel’s chemical composition using corrosion-resistant alloys or ion implantation. By exposing the metal in an environment mixed with corrosion inhibitors, corrosion resistance can be improved as well.  Another way is by applying barrier coatings on the stainless steel surface thus adding protection against corrosion forming elements.  Corrosion resistance can also be improved by cathodic or anodic protection of stainless steels.

            This research paper explains the various methods of improving corrosion resistance in stainless steels based on available literature.

Corrosion-Resistant Alloys & Ion Implantations

Duplex Stainless Steels

            A study of mechanical and corrosion properties of some stainless steel alloys was performed by Wei, Z., et al. (2008).  Three types of stainless steels were analyzed: austenitic stainless steels, ferritic stainless steels & the duplex stainless steels.  According to Wei,

            Austenitic stainless steels have good corrosion resistance and high toughness but   they have high sensitivity to local corrosion in chloride environment.

  Ferritic         stainless steels have good stress corrosion resistance, but they show low ductility.          Duplex stainless steels (DSS) are characterized by a dual phase structure with approximately equal volume fractions of ferrite and austenite, offering an attractive             combination of corrosion resistance and mechanical properties (cited in Solomon and        Devine, 1983).  (1)

Previous research and development have been directed toward the improvement of the duplex stainless steel by altering its chemical composition.  Notable are the works of Wang et al., Morello et al., and especially Outokumpu.

            The experiments performed by Wei et al., aimed at investigating the microstructure, mechanical and corrosion properties of duplex stainless steels, and the reasons for their good properties were also presented.  The pitting corrosion resistance of the specimens was evaluated by the critical pitting temperature or CPT measured using electrochemical tests.

            The specimens for ferric chloride corrosion test were ground with No. 600 silicon carbide sandpaper. Ferric chloride corrosion tests were carried out according to National Standard of the P.R.C. GB/T-17897-1999. The specimens for pitting corrosion test were ground with No. 1200 silicon carbide sandpaper. The pitting corrosion resistance was evaluated by critical pitting temperature (CPT) measurements using electrochemical tests.

The following are some of the details about the experiment:

            The test solutions, 1 mol/L NaCl, were made up from analytical grade reagents and           distilled water. An anodic potential of 750 mV SCE was applied and the solution   temperature was increased at a rate of 1 °C/min. The current density was recorded     simultaneously throughout the test. The CPT was determined from thermometry curve      as the temperatures where the current density exceeded 100 μA/cm2. The electrolyte             was kept being deaerated with pure nitrogen gas (N2) throughout the whole test. (1)

It was shown that duplex stainless steels have indeed a high resistance to pitting corrosion as confirmed by the rate of corrosion and CPT.  This is due to the dual phase structures containing nitrogen and molydenum, according to some researchers.  Earlier researchers however suggested that the high resistance of duplex steel against pitting corrosion depends on the content of Cr, Mo, and N.  Still others suggested the effect of elements like Mn, P and S.  Based on this theory, the pitting resistance equivalent number or PREN is defined by the following formula:

            PREN = %Cr + 3.3 x %Mo x %N – 1 x %Mn. (1)

A higher value of PREN indicates better pitting corrosion resistance.  Wei et al. finally concluded  that,

            DSS 2101 has high yield stress together with good ductility. Impact energy of 2101 at      20 °C is as high as 200 J along rolling orientation and declines with the decrease of     temperature. Meantime, impact energy along T–L is 80–90 J lower than the values   along rolling orientation. Ferric chloride corrosion test shows that rate of corrosion of           2101 and AISI 304 is 2.67 g/m2 × h and 4.61 g/m2 × h, respectively. CPT of 2101 is        determined to be about 20.0 °C, being obviously higher than 4.6 °C of AISI 304. PREN of 2101 and AISI 304 calculated by Thermo-Calc are 22.4 and 18, respectively.      These indicate that 2101 has better pitting resistance than AISI 304. (1)

Alloying Elements in Passivation

            Hashimoto, K., et al (2006) presented work showing the role of alloying elements in enhancing the corrosion resistance of stainless steels using the principle of passivity. According to Hashimoto,

            Passivation is the sudden decrease in corrosion rate of actively dissolving metal as a          result of an increase in oxidizing power of the environment.  However, the prerequisite for the use of metallic materials is spontaneous passivation in the given environment.  Spontaneous passivation is generally based on the presence of the air-formed film, which is stable or can be converted to the stable passive film before   serious corrosion degradation in the given environment.  The open circuit potential of the material should be in the passive region of effective alloy components having the       high passivating ability.  (2)

Hashimoto further states that a passive film should have the ability to act as a diffusion barrier because corrosion occurs through diffusions of cations and anions in the passive film.

            Surface analysis of stainless steels based on studies performed by earlier researchers became available in the 1970’s but the results obtained is dependent on the environment where the specimens were exposed.  This made it difficult to evaluate the role alloying elements have in corrosion reduction.  In acidic environments however, chromium-enriched oxyhydroxide passive film was identified.

            In the research performed by Hashimoto et al, specimens containing molybdenum and chromium as alloying elements were observed after the specimens were exposed in air and immersed in an acidic solution.  The high chromium content prevented pitting corrosion while the addition of molybdenum was observed to be beneficial.  The double oxyhydroxide passive films was found to have the highest stability in aggressive acid solutions and specimens containing chromium, molybdenum and tantalum were observed to have high corrosion resistance.

The following are some of the details about the experiment mentioned in Hashimoto’s research:

            The specimen was mechanically polished by silicon carbide paper in air-saturated cyclohexane, exposed to air for 30 min and immersed in 1 M HCl. The specimen was       spontaneously passive, and the open circuit potential was about −0.05 V vs. SCE in        the passive region of molybdenum. After immersion for 30 sec the specimen was washed and transferred through air to the X-ray photoelectron spectrometer. From the             observed spectrum the metallic and hexavalent state molybdenum can be identified. It       was found after detailed separation (cited in Kim and Winograd, 1974) that the oxide film consisted of tetravalent, hexavalent and some pentavalent molybdenum. The            ratio of Mo4+, Mo6+ and Mo5+ were about 5:3.5:1.5. As shown in Fig. 5, (2)

            prolonged immersion led to increase in relative amounts of both chromium and      molybdenum and total mass of Mo4+ in the film in spite of decrease in the film      thickness. The increase in the amount of Mo4+ with time suggested that Mo4+ was           not directly exposed to the solution at high potentials in the transpassive region of         molybdenum. (2)

Hashimoto et al. concluded that,

            Passivity of alloys containing corrosion-resistant elements were reviewed.  Chromium      and valve metals form stable oxyhydroxide films even in aggressive hydrochloric      acids. In particular, solid solution alloys containing chromium and tantalum form a      double oxyhydroxide film Cr1−xTaxOy(OH)3+2x−2y, which provides no corrosion weight loss even in 12 M Hcl.  Molybdenum forms a passive MoO2 film in the active   region of stainless steels and hence decreases the active dissolution current. However,       air exposure results in oxidation of tetravalent molybdenum to the hexavalent state.        Thus the surface analysis of the specimen transferred to the analytical apparatus        through air after polarization in the active state of the alloys results in the detection of        a large amount of mostly hexavalent molybdenum.  In the passive region of transition         metals and valve metals, molybdenum is generally in the transpassive state and      dissolved. However, if the outer oxyhydroxide film is stable the inner MoO2 film is   protected by the outer oxyhydroxide film and the MoO2 film acts as the effective barrier against diffusion of matters through the film. Thus the passive current density           of 30Cr–2Mo ferritic stainless steel is more than two orders of magnitude lower than     that of 30Cr steel without molybdenum in 1 M Hcl. (2)

Nickel Implanted Stainless Steel

            Feng, K. et al.  (2008) investigated on nickel implanted 316L steel as a bipolar plate for a PEM fuel cell.  According to Feng,

            In contrast to graphite, metallic materials can be used as bipolar plates because of their good mechanical strength, high gas impermeability, low cost and ease of        manufacturing (cited in Hornung & Kappelt, 1998, Spah et al, 2002, Peleen et al,            2002, Wang & Turner, 2004, Sweikart & Turner, 2003, Guiheen et al, 2001,   Matsumoto et al, 2001, Nikam & Reddy, 2005, Nikam & Reddy, 2006 and Huang et        al, 2003).  Stainless steel is considered as one of the promising candidates.  However,            surface modification is necessary for stainless steel to be used for the bipo- larplate in PEMFC.  (3)

One such modification on stainless steel is done by ion implantation.  According to Feng,

            Ion implantation is widely used to modify surface properties such as the mechanical          and tribological properties. Unlike in other thermodynamics limited processes such as diffusion and phase transformation, the quantity of the elements that can be ion implanted is not restricted by solubility and phase diagrams

            (cited in Xu et al,  2006).  (3)

The experiment performed by Feng et al. involved using stainless steel 316L plates as specimens.  The ion implantation was done in an improvised multifunctional ion implantation aparatus.  Proportional amounts of Nickel and Chromium were implanted in the steel specimens. Ni and Cr contained in the bare 316L and implanted 316L were then analyzed.

The following are some of the details about the experiment:

            It was found that corrosion resistance of the Ni implanted stainless steel 316L has improved due to the formation of Ni on the steel surface after ion implantation.  Feng et al. concluded that,

            The 316L stainless steel was nickel implanted for 1 × 1017, 2 × 1017, 3 × 1017, (3)

            4 × 1017 and 5 × 1017 ions cm−2, respectively. The potentiodynamic and             potentiostatic test measured at 80 °C in 0.5 M H2SO4 with 2 ppm HF solution, and   the ICP results, which are in agreement with each other, demonstrate that the         corrosion resistance is improved by the formation of a Ni-rich surface layer with ion           implantation. In particular, when nickel implanted with a dose of 3 × 1017 ions cm−2,      the corrosion potential (Ecorr) moved to about −0.05 V versus SCE in the accelerate   anode environment and the passivation current density reduced to 7 μA cm−2,   indicating that nickel implantation could greatly improve the corrosion resistance of            SS316L in both anode and cathode environment. However, when the ion implantation    was carried out for increased dose (for 4 × 1017 and 5 × 1017 ions cm−2), the current      density increased, indicating that the flaws and defects caused by the ion

            implantation deteriorated the Ni-rich layer’s corrosion resistance. The nickel           implantation could markedly decrease the interfacial contact resistance of SS316L due             to the reduction in passive layer thickness. (3)

Molybdenum Ion Implanted Stainless Steels

            Mottu, N. et al.  (2004) experimented on the structure and composition effects on pitting corrosion resistance of austenitic stainless steel after molybdenum ion implantation. According to Mottu,

            Molybdenum ion implantation could harden the surface of steel (cited in Sharkeev et        al, 1999 and Sharkeev et al, 2002) and increase the pitting corrosion resistance of           stainless steel (cited in de Buchere et al, 1996 and Isaacs & Huangs, 1996). (4)

In their experiment, Mottu et al. used austenitic stainless steel bars as specimens.  Mo was implanted at room temperature. The following are some of the details about the experiment,

            Implanted and unimplanted surfaces were examined before and after the (4)

            corrosion attack with a Hitachi S4200 scanning electron microscope (SEM), using an       acceleration voltage of 20 kV. Secondary electron mode was used to characterize the     surface morphology and EDX mode (Energy Dispersive X-ray Analysis) to determine     the elemental composition. (4)

Results of the experiment show that the increase in Mo implantation dose has a beneficial effect on corrosion resistance.  Mottu et al. concluded that,

            As we saw previously, the samples had a complex behavior with respect to corrosion        resistance and different aspects of electrochemical behavior of the samples have to be       considered.  The samples can be then divided into two groups, the transition dose is           8×1015 ions cm−2:• up to a dose of 8×1015 ions cm−2. Mo implantation induces: –an       improvement of the stability of the oxidation layer; –an improvement of pitting             corrosion resistance; –an improvement of the repassivation of pits created by         electrochemical attack. The size of the pits and the proportion of surface area covered         by pits are the same as those observed for unimplanted samples.  The electrochemical   behavior of these implanted samples is better than this of unimplanted samples. This     has to be connected to all the characteristics of the samples.  Mo implantation leads to         an increase of molybdenum concentration in the implanted layer and consequently in        the passive layer. The beneficial effect of molybdenum for corrosion resistance

            has been already demonstrated by many authors (cited in Latha et al, 1997, Brookes

            et al, 1990 and Sugimoto & Sawada, 1976) when molybdenum is introduced as    alloying element. They have demonstrated that the presence of molybdenum in the    passive layer plays a fundamental role.  The surface topography can play a major role      in pitting corrosion (cited in Briggs & Seah, 1990) since holes, strays and surface       defects are preferential sites where     pitting corrosion takes place. We evaluated (4)

            the surface topography by determining the RMS roughness. The implanted samples          are rougher than the unimplanted sample. Consequently if we consider only the            roughness the electrochemical behavior must be worse and consequently another    parameter with a higher influence must explain the   better electrochemical behavior of    these implanted samples.  Implanted samples have    the same structure (austenite) as         the unimplanted samples. The lattice parameter of austenite only increases starting

            from a dose of 8×1015 ions/cm2.  Implantation dose increase induces: –a reduction in       the stability of the oxidation layer, –a reduction in    the pitting corrosion resistance, –       an improvement of the repassivation of the pits created by electrochemical attack, –an      increase in the size and density of the pits.  In the same time, the increase of             molybdenum implantation dose increases the molybdenum concentration in the             implanted layer and consequently in the passive layer. This must be have a            beneficial effect of corrosion resistance, as it has been previously discussed.  The         increase of implantation dose has a high influence on the observed structure of the      implanted layer. Indeed, in this group, ferritic structure appears and then     amorphization takes place. Austenitic structure has a better corrosion resistance than          the ferritic structure. Moreover, the amorphization process is not favorable to better     corrosion resistance since it creates defects and is responsible for degradation of the o    xide layer stability. Size of pits and proportion of surface area covered by pits    increase.  Improvement of the repassivation of the passive   layer  observed in this             group has to be related to the increase of Mo concentration in the passive layer. (4)

Corrosion Inhibitors

            Ahmad, Z.  (2006) presented a paper on corrosion control by inhibition.  Ahmad   stated that,

            Corrosion phenomena are well-known in the petroleum industry and cause a         maximum damage to oil field equipment. Significant corrosion protection efforts have     been made by petrochemical industries to prevent corrosion damage. The practice of    corrosion prevention by adding substances which can significantly retard corrosion           when added in small amounts is called inhibition. Inhibition is used internally with             carbon steel pipes and vessels as an economic control alternative to stainless steels            and alloys, and to coatings on non-metallic components. One unique advantage is that       adding inhibitor can be implemented without disruption of a process. The addition of        an inhibitor (any reagent capable of converting an active corrosion process into

            a passive process) results in significant suppression of corrosion. (5)

Chromate in LiBr

            A research was done by  Muñoz, A. et al.  (2006) on the effect of chromate in the corrosion behavior of duplex stainless steel in LiBr solutions.  According to  Muñoz,

            ..chromates are very good inhibitors of pitting corrosion in stainless steels (cited in    Igual-Muñoz et al, 2004).  It is generally assumed that in acid media, corrosion         inhibitors adsorb on the metal surface which results in a structural change of the          double layer and a reduced rate of the electrochemical partial reaction (cited in   Popova et al, 2003). (6)

Muñoz further stated that,

            The advantage of the Duplex alloys in media that contain halide, such as chloride or             bromide, resides in the content of δ-ferrite, which promotes combined effects due to             electrochemical and mechanical factors. (6)

The experiments performed by Muñoz et al. Involved using commercial duplex stainless steel rods, AISI 2205 as test specimens.  The specimens were tested in aqueous LiBr solution.

The following are some of the details about the experiment:

            Two different electrochemical tests have been conducted in LiBr aqueous solutions:             cyclic potentiodynamic curves, performed to assess the corrosion resistance by        recording anodic and cathodic currents, and electrochemical impedance spectroscopy           (EIS) testing, performed to evaluate surface’s performance. All measurements were            carried out using a potentiostat Solartron 1287. The cell was a three-electrode           horizontal cell, with platinum counterelectrode and a silver/silver chloride, 3 M KCl,       reference electrode. Experiments have been conducted at room temperature. Dissolved oxygen was removed from the LiBr solutions by bubbling nitrogen for 10             min before each anodic polarization measurement; a constant nitrogen flow was      constantly maintained inside the electrochemical cell during the whole test. Before          measurements were made, open circuit potentials were obtained for 60 min. The     potentiodynamic measurement rate was 0.5 mV/s, started from −1000 mV and finally moved in the anodic direction. Current corrosion density (icorr) and corrosion            potential (Ecorr) were obtained from the polarization curves using the Tafel slopes.             The potential at which the current density exceeded 100 μA/cm2 was defined as the             pitting potential (Ep). For the cyclic scans, the potential scan was reversed when the             current reached a value of 5 mA/cm2. At least three repeat experiments were carried          out for all specimens. Once cyclic curves were determined, pitting potentials, passivation current density (ip) and repassivation potentials (Erp) were calculated as     characteristic parameters for defining the behavior of a material in a corrosive

            media. (6)

The results of the experiment showed that it is possible to obtain an optimum ratio of chromate bromide which minimizes the corrosion effects.  Muñoz et al. concluded that,

            Chromate effect on corrosion behavior of a DSS in LiBr media was studied by OCP             measurements, EIS, potentiodynamic cyclic curves and image analysis.  Evaluation of         the electrochemical parameters for the DSS allowed to established a possibly       optimum ratio chromate/bromide which minimizes the corrosion effects.  Polarization           curve and EIS tests allow to point out that in LiBr media, with and without chromate,         the most suitable corrosion mechanism includes the double layer and the passive film      formed on the alloy surface. Chromate presence favors the passive film formation by          increasing the resistance of that passive film. The resistance of the passive film    increases with the LiBr concentration although the thickness diminishes when the    ratio chromate/bromide does not exceed the value of 0.011. On the other hand, the            charge transfer resistance decreases with LiBr concentration demonstrating the high sites of pit initiation on the DSS surface due to the bromide penetration in the passive     film. (6)

Borate in Low-level Radioactive Water Containing Chloride

            Bellanger, G.  (2006) presented a study on inhibition of localized corrosion for stainless steels in low-level radioactive water containing chloride  On the treatment of low-level radioactive water like tritiated water used in industrial installations, Bellanger stated that,

            The non-negligible amount of tritiated water at low-level is stocked into containers            made of steel before treatment. Amounts of tritium and energy released deposited on       the oxide surface of steel are lower than 10−10 atoms cm−2 and 3 × 102 MeV cm−2     s−1 respectively. At this low activity level, influence of radiation into oxide for           degradation is negligible (cited in Bellanger & Rameau, 1003).  There is only             ionization of oxide layer by the energy released during tritium decay. During (7)

            storage, addition of borate or silicate anions buffers the tritiated water to slightly    alkaline pH. Adsorption of these anions plays a significant role in corrosion inhibition. These anions modifying surface pH inhibit pitting corrosion, and the extent of     inhibition depends upon the type of stainless steel and the concentration of

            inhibitor. (7)

In the experiments performed by Bellanger, super-duplex steel immersed in low-level tritiated containing chloride was evaluated.  The corrosion inhibitor used was borate.  After a series of tests in the presence and absence of cloride and inhibitors, Bellanger concluded that,

            Borate is not decomposed by radiolysis in low-level tritiated water, therefore it can be       used as inhibitor of pitting corrosion by chloride. Results show the high oxide layer           stability when the concentration of inhibitor is enough high.  Borate acts from the       active region by adsorption and buffering. At large coverage, the surface remains    passive and free of pitting attack and pit grown until the transpassive region is          reached. Nevertheless borate has a lack in the inhibition ability to repair attack when         its concentration is too low. This indicates that the passive layer where a pit grown     was initiated, small addition of borate does not totally repair the defect. (7)

Austenitic Steel Chemically Treated with Cerium

            As an extension of previous work involving Cerium, Lu, Y. & Ives, M.  (1995) worked on the chemical treatment with cerium to improve the crevice corrosion resistance of austenitic stainless steels.  A study was made on the cathodic electrode process and its inhibition by cerium salt treatment on austenitic stainless steels in a simulated seawater solution.

            Based on the results of the experiment performed by Lu & Ives, it was shown that cerium improved the localized corrosion resistance, particularly the crevice corrosion resistance of stainless steels.  According to  Lu & Ives,

            The effect of cerium ion implantation in improving the localized corrosion resistance         of stainless steels was demonstrated earlier. The work has now been extended to study            the effect of cerium salt solution treatment. Rotating disc assemblies were employed            to study the cathodic electrode process and its inhibition by cerium salt treatment on        austenitic stainless steels in a simulated seawater solution. The reduction of oxygen    and protons on both untreated steels is shown to be controlled by the mass transport             processes in solution. Cerium treatment effectively inhibits the cathodic reduction of         oxygen which is controlled primarily by charge transfer at the electrode. The         overvoltage for cathodic reduction of protons increases after the cerium treatment and         the electrode reaction is controlled by both the mass transport process in solution and           the charge transfer on the electrode. As a result of inhibition of the electrode             processes, cerium improves the localized corrosion resistance, and in particular the c         revice corrosion resistance, of stainless steels. This is supported by results obtained f        rom electrochemical measurements and crevice corrosion tests both in the laboratory      and at a seawater test site. (8)

Nano-Titania Coated Stainless Steels

            Yun, H. et al.  (2007) performed a study on the N-, S- and Cl-modified nano-TiO2 coatings for corrosion protection of stainless steel. According to Yun et al,

            Chromium-containing compound coatings have been successfully used for the      corrosion protection of steel for many decades. However, environmental laws in many       countries have imposed severe restrictions on chromate use due to its high toxicity and consequent environmental hazards. Many alternatives to find suitable             replacements for chromium-containing materials have been (9)

            explored (cited in Kasten et al, 2001, Hamdy & Butt, 2006, Jing et al, 2006 and Hu et      al, 2006).  One of the   most effective methods is to coat the surface of steel with     protective ceramic compounds, such as transition metal oxides, nitrides, carbides or   silicides (cited in Garbajal et al, 2001).  Among these coatings, the TiO2 coating    prepared by a sol–gel method is probably the most   commonly used for steel         protection (cited in Shen et al, 2005, Meinert et al, 1998 and Masalski et al,.1995)      TiO2 is also well known as an n-type semiconductor for photo-catalysis and an      ultraviolet (UV) light sensitive hydrophilic agent (cited in Yuan & Tsujikawa, 1995,        Nakajima et al, 2001 and Diebold, 2003).  Recently, the applications of the TiO2   coating for cathode protection of metals under UV illumination have been reported in        literature (cited in Huang et al, 1997, Mahmoud et al, 2005 and Ohko et al, 2001).              When a metal coated with a thin TiO2 coating is exposed to UV irradiation, electron–  hole pairs are generated in the TiO2 coating. The photogenerated electrons can be transferred to the metal substrate thereby making its electrode potential more negative       than its corrosion potential. Furthermore, in this case, titania does not get consumed           because the anodic reaction is not the decomposition of TiO2 itself but the oxidation of      water and/or adsorbed organic species by the photogenerated holes, unlike a           sacrificial-type cathode protection (cited in Li et al, 2005, Subasri et al, 2006). However, due to the depletion of organic substances with time, the practical          application of this technology will be somewhat limited. On the other hand, there        exists an inherent disadvantage of using a pure TiO2 coating for such anticorrosion        applications, which is that the photo effect becomes void once the UV illumination          stops shining on the TiO2 coating. Thus, investigating of the TiO2 coating as protective             materials for steels without the photo effect is of considerable practical interest. (9)

            In the experiments conducted by Yun, H. et al, 316L stainless steel was used as specimen.  The nano-TiO2 coating was applied by dip-coating method. The coatings were modified accordingly in various solutions of N, S, and Cl.  Based on the analysis and results,  Yun et al concluded that,

            ..the N-modified TiO2 coating shows a remarkable improvement in corrosion         protection of steel in 0.5 M NaCl. A dense and uniform and well-crystallized TiO2          coating attained by addition of concentrated HNO3, is beneficial for enhancing the           corrosion resistance. And the existence of nitrogen in the TiO2 coating is valid to        improve the coating structure and its corrosion protection as well. It is expected that             the TiO2 coatings modified by anions can be used as excellent protective coatings for        metals in various applications. (9)

Blend Coatings to Corrosion Inhibition

            Moraga, G. et al (2006) presented a work on poly(2,5-dimethoxy-aniline)/fluoropolymer blend coatings to corrosion inhibition on stainless steel. According to Moraga et al,

            Heterogeneous blends of a soluble substituted polyaniline, poly(2,5-dimethoxy           aniline) (PDMA), and two fluoropolymers, poly(vinylidene fluoride) and          poly(tetrafluoroethylene-co-vinylidene fluoride-co-propylene) were prepared by       casting. The use of this ternary system allowed the compatibilization of the polymer          mixture and the improvement of the adhesion to the substrate of stainless steel. Three         compositions with increase content of PDMA were investigated and compared with          PDMA coating behavior. The superficial composition of the coatings was             characterized by wide-scan X-ray photoelectron spectroscopy, which indicated that             the steel surface is totally covered by the polymers. Chronopotentiometry at (10)

            zero  current systematic study was used to determine the open circuit potential (OCP).        After 7 days immersion in saline solution, PDMA and the three blends showed similar          OCP values of −170 mV (reduced state), which is shifted to more positive values in relation to the bare steel. Electrochemical impedance spectroscopy was used to         characterize the polymeric coatings in the absence of active corrosion at the         metal/coating interface. The blends showed higher Rct compared to PDMA (102–103      times) and this result was interpreted as a contribution of barrier of fluoropolymers.           Salt spray test was used to evaluate the corrosive resistance of coatings and blends in      aggressive conditions during 1000 h. Energy dispersive spectrometer semi-     quantitative chemical analysis was performed on the coatings after the test and         indicated that blends show superior resistance to the fog test in relation to the PDMA           coating. Therefore, the blends PDMA/fluoropolymer behave as an electrochemical         barrier as well as a physical barrier showing superior corrosion inhibition in relation           to PDMA. (10)

In the experiment performed by Moraga et al., 304SS stainless steel specimens were used to prepare the samples.  PDMA and blend polymeric solutions were deposited on the specimens.

The coated stainless steel specimens were exposed to salt fog evironment.  Based on analysis and results Moraga et al. concluded that,

            after the test and indicated that blends show superior resistance to the fog test in      relation to the PDMA coating. Therefore, the blends PDMA/fluoropolymer behave as          an electrochemical barrier as well as a physical barrier showing superior corrosion            inhibition in relation to PDMA. (10)

Polyindole on 304-stainless steel

            Düdükcü, M. & Köleli, F. (2006) worked on the electrochemical synthesis of polyindole on 304-stainless steel in LiClO4–acetonitrile solution and its corrosion performance.  According to Düdükcü, M. & Köleli, F.,

            Polyindole is an electroactive polymer, which can be obtained after anodic oxidation            of indole in various electrolytes; electrochemical oxidation of indole in LiClO4    containing acetonitrile electrolyte medium gives an electrochromic polymer film with good air stability. (11)

Düdükcü & Köleli further stated that,

            Stainless steels are used in various applications for their corrosion resistances. The   corrosion resistance of stainless steel depends on a very thin surface film, called oxide    film. Although the formation of a film of chromium oxide is effective for protecting           stainless steel, the corrosion advances rapidly when localized damage on this passive            film occurs (cited in Pourbaix, 1874). This localized dissolution of an oxide in specific aggressive environments is one of the most common and catastrophic causes   of failure of metallic materials. (11)

One of the aims of this work by Düdükcü & Köleli is to determine the performance of polyindole coated stainless steels.  In their experiment,  stainless steel culindrical rods were used as specimens.  Electrochemical synthesis of plyindole was carried out on a stainless steel rod in a container containing  LiClO4–acetonitrile solution.  Based on analysis and results,  Düdükcü & Köleli concluded that,

            The synthesis of PIN film was succeeded on SS electrode by using cyclic         voltammetry technique. Adherent and stable polymer film was obtained on the        electrode. The corrosion protection ability of PIN film was investigated by using EIS             and anodic polarization curves. PIN coating shifted the electrode potential towards      more positive potentials and exhibited an excellent protective behavior for an (11) important exposure time in chloride solution. The anodic polarization curves, Eocp             and impedance measurements showed that PIN coating promises to be a good          candidate for corrosion protection of SS. (11)

Conversion layer and Titanium dioxide

            Bamoulid, L.et al (2008) proposed an efficient protection of stainless steel against corrosion: combination of a conversion layer and titanium dioxide deposit.  Bamoulid et al. stated that,

            In the present work, a novel process has been developed to improve the corrosion             properties of ferritic stainless steels. Titanium oxide coatings have been deposited           onto stainless steel by sol–gel process after a pre-functionalization of the substrate in a        conversion bath. Gel titania was prepared by hydrolysis of a titanium butoxide        through a sol–gel process. Duplex systems “conversion layer/uniform TiO2 coating”             have been prepared on stainless steels using a dipping technique and thermal post- treatments at 450 °C. The preparation of sol–gel coatings with specific chemical   functions offers tailoring of their structure, texture and thickness and allows the fabrication of large coatings. The morphology and structure of the coatings

            were analysed using scanning electron microscopy with field effect gun (SEM-FEG),       Mass spectroscopy of secondary ions (SIMS) and X-ray diffraction (XRD). The             anticorrosion performances and the ageing effects of the coatings have been evaluated   in neutral and aggressive media by using several normalized tests.

            The results show that the conversion layer was not sufficient to protect steel but sol–         gel TiO2 coatings, anchored on the metal substrate via the conversion layer, show    good adhesion with the substrate and act as a very efficient protective barrier against         corrosion. So, duplex layers with TiO2 nanoparticle coatings on steels exhibit (12)

            an excellent corrosion resistance due to a ceramic protective barrier on metal surface.        Analysis of the data indicates that the films act as geometric blocking layers against            exposure to the corrosive media and increase drastically the lifetime

            of the substrate. (12)

1,2,3-benzotriazole in acidic media

            Satpati, A. & Ravindran, P.  (2008) performed an electrochemical study of the inhibition of corrosion of stainless steel by 1,2,3-benzotriazole in acidic media.  According to Satpati A. & Ravindran,

            The mechanism and efficiency of corrosion inhibition using 1,2,3-benzotriazole    (BTAH) as the inhibitor on stainless steel corrosion in sulphuric acid medium were            studied. Electrochemical impedance spectroscopy (EIS) as well as the       potentiodynamic polarization was used for the quantification of inhibition efficiency        and determination of the mechanism of interaction of the metal surface with the     inhibitor molecule. Under the present experimental condition 1,2,3-benzotriazole was          found to be an efficient inhibitor for the acid corrosion of austenitic stainless steel and         the inhibition efficiency of up to 97% was obtained. In presence of inhibitor there          is a diffusional contribution on the mechanism of the corrosion process was observed             in impedance measurements at low frequency region. Effect of temperature on the             mechanism and also on the efficiency of the corrosion inhibition process was studied        using the Arrhenius approximation of the rate law. Polarization experiments were     performed at different solution temperatures. Thermodynamic parameters for the     corrosion process were obtained and interpreted. The adsorption of the inhibitor,     BTAH, on the stainless steel surface in the acid medium (0.1 M H2SO4) obeyed the       Langmuir adsorption isotherm. Present results were discussed and compared with (13)

            the similar results reported by some other groups. (13)

2-mercaptobenzoxazole

            Refaey, S., et al.  (2004).  worked on the inhibition of stainless steel pitting corrosion in acidic medium by 2-mercaptobenzoxazole.  According to Refaey, S., et al.,

            The corrosion behavior of stainless steel samples (304L and 316L) in HCl and H2SO4     solution has been studied using potentiodynamic, cyclic voltammogram, EDX and        scanning electron microscope (SEM) techniques. The inhibition characteristics of 2-          mercaptobenzoxazole (MBO) on 316L stainless steel (316L SS) in HCl solutions were      investigated at different temperatures (25, 40, 50 and 60 °C). MBO compound has             proven to be efficient inhibitors for general and pitting corrosion of 316L SS

            in HCl solution. The inhibitive property of MBO may be argued to the formation of          very low soluble bis-benzoxazolyl disulfide (BBOD) layer and a compact Fe–MBO            complex film on the electrode surface. Some samples were examined by scanning electron microscope. The inhibition efficiencies increased with the increasing of MBO   concentration but decreased with increasing temperature. The activation energy and      thermodynamic parameters were calculated. (14)

Oxyanions tungstate and molybdate.

            Alentejano, C., & Aoki, I.  (2004).  worked on the localized corrosion inhibition of 304 stainless steel in pure water by oxyanions tungstate and molybdate.  According to Alentejano & Aoki,

            The aim of this work is to study the efficiency of the oxyanions tungstate and            molybdate as pitting corrosion inhibitors for 304 SS in deaerated pure water typical            from nuclear reactors circuit for vapor production in the presence and absence

            of chloride ions. The following electrochemical techniques were (15)

            employed: open circuit potential (OCP) monitoring and potentiodynamic or cyclic   polarization curves, at 90 °C and room temperature, with static and rotating disk electrodes. The experiments were performed in pure water with or without 200 ppm         sodium chloride (NaCl) in absence and presence of 10−4 and 10−3 M ammonium     molybdate [(NH4)6Mo7O24·4H2O] or sodium tungstate (Na2WO2·2H2O).

            The addition of molybdate and tungstate improved the quality of 304 SS passive layer         in pure water at both temperatures, but at 90 °C all the OCP observed were more     negative than at room temperature. In the presence of chloride ions, the corrosion      inhibitors raised the 304 SS pitting potential to more positive values. The increase of            pitting potentials occurred for increasing concentrations of tungstate and molybdate.          Otherwise, the increase of temperature decreased the pitting potential, that is, the             efficiency of the oxyanions decreased with temperature increase. The influence of    hydrodynamic conditions was also investigated. It was observed that molybdate             oxyanion have always showed better efficiency than tungstate. (15)

Chromate and Molybdate

            Ilevbare, G. & Burstein, G.  (2003)..presented a study on the inhibition of pitting corrosion of stainless steels by chromate and molybdate ions.  According to  Ilevbare & Burstein,

            The inhibitive effects of chromate and molybdate on pitting corrosion in stainless steel         AISI 304 and AISI 316 were studied in acidified chloride solution. The results        presented show that these known inhibitors affect both the nucleation of pitting and         metastable pitting by reducing the numbers and sizes of these events. This makes     attainment of stable pit growth more difficult. (16)

Ilevbare & Burstein conducted potentiostatic and potentiodynamic experiments.  Based on the analysis and results, Ilevbare & Burstein concluded that,

            Chromate and Molybdate affect pit nucleation by deactivating the sites at which they        occur and by reducing the sizes of those that occur. As a result, metastable pits are           more difficult to develop from these nucleations and a reduction in the number of            metastable pits occurs. This causes a reduction in the probability of developing stable         pits, and an increase in pitting potential results. There is a correlation between the     numbers and sizes of nucleations, their ability to transfer to metastability, and stable           pitting. The lower passive current density displayed by the steels in the presence of           the inhibitors compared with pure HCl suggests that passivity is a little more stable in       the presence of the inhibitors. (16)

Barrier Coatings

Composite Thermal Barrier Coatings on Stainless Steel

            A research was conducted by Zhiang, J. & Kobayashi, A.  (2008) on the corrosion resistance of Al2O3+ZrO2 thermal barrier coatings on stainless steel substrates.  According to Zhiang and Kobayashi,

            Spallation is the overall result of all the thermal oxidation reactions in the coating     substrate interfaces. Although the coating stock materials Al2O3+ZrO2 have        intrinsically excellent oxidation resistance, pores in the composite coatings provide         the way for the oxidants in the ambience to access the interfaces. Therefore it is           essential that such defects are characterized by the anodic polarization measurement, which is commonly used for material corrosion analysis. (17)

Using a gas tunnel type plasma spraying setup, Zhiang and Kobayashi performed an experiment by spraying Al2O3+ZrO2 composite powder on stainless steel SUS304 plates.       Analysis and results show that the composite coatings sprayed on the specimens produced an improvement in the corrosion resistance of stainless steels.  Zhiang and Kobayashi concluded that,

            The corrosion behavior of the Al2O3 + ZrO2 composite coatings sprayed on SUS304             substrates by gas tunnel type plasma showed improvement in the oxidation resistance         of the coated samples. The conclusions are as follows: 1) The corrosion potential of             the samples is raised by the ZrO2 + Al2O3 composite coatings. Thicker coatings and a        high mixing ratio of Al2O3 lead to a higher corrosion potential. The corrosion          potentials of the coatings with different alumina mixing ratios showed different        descending slope with coating thickness.  2) On the other hand, the corrosion current           density of the coated samples decreased with coating thickness at almost the same    slope for different coatings containing alumina. However, the current density shows          inconsistent tendencies with coating porosity for thicker and thinner coatings, because     the current tendency for thicker coatings is more strongly decided by oxidation on the    interfaces and is less dependent on the coating porosity.  3) Based on the similarity          between anodic reactions and the thermal oxidation reactions on the interfaces, lower   corrosion potential and lower corrosion current density indicate a higher resistance for             thermal oxidation of the coating interfaces and a lower thermal oxidation rate. Thus, a higher alumina mixing ratio and higher thickness of the ZrO2 + Al2O3 composite             coatings, by way of lowering the porosity and increasing the gradient of coating       porosity, help lowering the oxide layer growth. (17)

Corrosion Protection by Polymer Coatings

            Tan, C. & Blackwood, D.  (2003) worked on corrosion protection by multilayered conducting polymer coatings on carbon steels and stainless steels.  According to Tan & Blackwood,

            Multilayered coatings, consisting of combinations of the conducting polymers            polyaniline (Pani) and polypyrrole (Ppy), were galvanostatically deposited on to both      carbon steel and stainless steel. Potentiodynamic polarisation was used to assess the          ability of these copolymers to provide an effective barrier to corrosion in chloride   environments. For carbon steel the performance of these multilayered coatings on      carbon steel were not sufficiently better than for single Pani coatings to justify their    more complicated deposition procedures. However, in the case of stainless steels the new multilayered coatings proved to be significantly better than previously reported        single Pani coatings, especially at protecting against pitting corrosion. (18)

In the experiments performed by Tan & Blackwood, carbon steel and 304L stainless steel rods were used as specimens.  In the first set, a Pani film was coated on the metal surface and topped by a Ppy film (Pani/Ppy).  In the second set, Ppy film was coated on the metal surface and topped by a Pani film (Ppy/Pani).  Subsequently, three different combinations of Pani and Ppy were coated on both carbon steel and 304L stainless steel.  Analysis and results show that  the multilayered polymer coatings had little effect on the corrosion resistance of carbon steel.  On the otherhand, application to 304L stainless steels showed improved corosion resitance.

Tan & Blackwood concluded that,

            Three different combinations of Pani and Ppy have been successfully coated onto both     carbon steel and 304L stainless steel. The morphology of the various coatings    appeared to be independent of the nature of the underlying metal. However, the   strength of the adhesion of the multipolymer coatings and their ability to provide            corrosion protection appeared to be substrate specific. In the case of carbon steel the             performance of these multilayered coatings on carbon steel were not sufficiently

            better than for single Pani coatings to justify their more complicated deposition (18)          procedures. In the case of stainless steels the new multilayered coatings proved to be         significantly better than previously reported single Pani coatings, especially at    protecting against pitting corrosion. This may be due to the multipolymer coatings            acting as better chemical barriers, trapping chloride and thus preventing it from             attacking the stainless steel substrate. The degree of protection, as well as the         morphology of the coating, strongly depended on the order in which the two    conducting polymers were coated. By far the best performance was achieved by            depositing a Ppy underlayer and a Pani top layer (Ppy/Pani), which both completely      eliminated pitting and reduced the corrosion rate by about a factor of 2000. This    coating had a closed packed morphology, suitable for acting as both physical and            chemical diffusion barriers, and was believed to be the most conductive and hence           able to provide the best electronic barrier. Likewise this Ppy/Pani coating also had the            strongest adhesion to the stainless steel substrate, although only by about 0.2 N and        the strength of its adhesion was less than half the value of a typical paint film. Overall       this suggests that the ability of a conducting polymer film to act as electronic and         chemical diffusion barriers are more important in providing corrosion protection than          its ability to act as a physical barrier. (18)

SiO2 PACVD Coated Stainless Steels

            Pech, D. et al. (2008) investigated the electrochemical behaviour enhancement of stainless steels by a SiO2 PACVD coating.  According to Pech,

            The remarkable corrosion resistance of stainless steels relies on their passive ability.             Indeed, the tenacious oxide passive film formed on stainless steels when exposed to a             gaseous or aqueous environment avoids any penetration of corrosive species into      the subsurface of the material (cited in Kim, 1999 and Sato, 1990). However, (19)

            its protective characteristic is sometimes not sufficient for some applications requiring         specific environments such as halideion-containing electrolytes. (19)

Pech further stated that,

            One effective way to overcome this problem is to coat the material surface with a thin       protective film (cited in Atik etal, 1994, Feng et al, 2003, Ibrahim et al, 2002,         Misaelides et al, 1997 and de Damborenea et al, 1995).  The protective effectiveness          of such coatings may be related to their barrier effect against the diffusion of water       and gaseous aggressive agents from the environment towards the metal surface (cited             in Masalski etal, 1999). (19)

In the experiments performed by Pech et al., M2 steel and austenitic stainless steel were used as specimens.  The specimens were coated with plasma assisted chemical vapour deposition (PACVD) silica-based coatings.  Based on analysis and results, part of the conclusions of  Pech et al. were as follows:

            When deposited on stainless steel, coated part keeps the beneficial passive behavior          inherent to the substrate. They evidence almost no corrosion. The silica based layer           behaves as a quasi perfect dielectric. The corrosion rate is then greatly reduced, and         the pitting resistance is improved. (19)

TiO2 nanoparticle coating

            Shen, G., Chen, Y., Lin, C.  (2005).  worked on the corrosion protection of 316 L stainless steel by a TiO2 nanoparticle coating prepared by sol–gel method.  According to Shen and Lin,

            A uniform and TiO2 nanoparticle coating on steels has been prepared using sol–gel method and hydrothermal post-treatments. The morphology and structure of the coatings were analysed using atomic force microscopy and X-ray diffraction. (20)

            The anticorrosion performances of the coatings in dark and under ultraviolet

            illumination have been evaluated by using electrochemical techniques. The influences          of coating thickness, pH and NaCl concentration on corrosion protection have been     examined as well. The results indicate that the TiO2 nanoparticle coatings on steels         exhibit an excellent corrosion resistance due to a ceramic protective barrier on metal surface in dark, and a photo-generated cathodic protection current under UV          illumination. The electrochemical impedance spectroscopy measurements provide         an explanation to the            increased resistance of nano TiO2 particles coated 316 L stainless steel against  corrosion. (20)

From their analysis and results of the experiments, Shen and Lin concluded that,

            We have prepared a uniform and smooth TiO2 nanoparticle coating on the 316 L by           the sol–gel method followed by a hydrothermal post-treatment. It is indicated that the    nano TiO2 particle coatings like ceramic protective films exhibit an excellent           corrosion resistance in the 0.5 mol L− 1 NaCl solution in dark. The influences of   coating thickness and pH have been examined, and it is indicated that the nano TiO2          particle coating with about 460 nm thickness exhibits the best corrosion resistance because of its perfect structure in the coating. It is evident that the corrosion potential         positively shifts, icorr decreases by 3 orders of magnitude, and corrosion resistance Rt             increases above 100 times after applying a TiO2 nanoparticle coating on 316 L          stainless steel comparing with bare steel in the same environment. Furthermore, under       UV illumination the photogenerated electrons result in a potential shift of metal       substrate to the corrosion immunity region. That is, an adscititious photogenerated    voltage is able to maintain a cathodic protection for the 316 L stainless steel in the    chloride containing environments. The TiO2 nanoparticle coating on metals (20)

            should            firstly play a role of good protective barrier, and also under light illumination

            the coating has an additional cathodic protective function to a metallic substrate. (20)

Stainless Steel Coated with Polyaniline

            Özyilmaz, A., et al.  (2004).  Investigated the corrosion behaviour of stainless steel coated with polyaniline via electrochemical impedance spectroscopy.  According to Özyilmaz, A., et al.,

            The electrochemical synthesis of polyaniline (PANI) was achieved on stainless steel   (316L) electrodes using cyclic voltammetry technique in monomer containing oxalic acid solution. Homogenous and strongly adherent films with different thickness were          obtained by applying two different potential ranges for the synthesis. The corrosion performances of these coatings were investigated by AC impedance spectroscopy. It was seen that polyaniline films provided better protection for long exposure times in           0.1 M HCl solution. The polymer films exhibited significant barrier property against             the attack of corrosive agents. It was found that 0.25–1.00 V was a more suitable      potential range compared with −0.20 to 1.65 V to obtain good protective

            coatings. (21)

Stainless steel coated by sol-gel ZrO2—CeO2 films

            Di Maggio, R., et al.  (1996).  worked on the dry and wet corrosion behaviour of AISI 304 stainless steel coated by sol-gel ZrO2—CeO2 films. According to Di Maggio et al.,

            ZrO2—CeO2 coatings were deposited on 304 stainless steel substrates by dip coating           in alkoxide solution. The coatings were transparent and continuous at thickness below            about 700 nm. The films were used as barriers against oxidation for 304 stainless steel           at high temperature. The best performance (no oxidation up to 16 h at 750 °C) was          obtained with a coating approximately 1 μm thick. The corrosion resistance of (22)

            the coated samples in an aggressive electrolyte (15 wt. % HCl solution) was studied by electrochemical impedance spectroscopy, a technique frequently used to test the       protective properties of organic and metallic coatings on metals. The protective action          of the deposited coatings was discussed as a function of the film thickness and

            quality. (22)

Anodic Protection

Galvanic coupling with platinum

            Bianchi, G., et al.  (1965).  worked on the anodic protection of stainless steel by galvanic coupling with platinum.  According to Bianchi, G., et al.,

            Stainless steel in sulphuric acid solutions at various concentrations and temperatures          is passivated and does not undergo corrosion if coupled galvanically to a sheet of           platinum when its surface area is in a definite ratio with the surface area of the       stainless steel. The ratio is 1 with 38 per cent aqueous sulphuric acid at room    temperature, but 100 with 52% H2SO4 solution at 75°C.  The values of the ratio   necessary for the passivation of stainless steel have been determined and related to the     results of the corresponding polarization curves for the anodic dissolution of stainless     steel and for the cathodic reduction of oxygen and of hydrogen ion in solution. The            results obtained by galvanic coupling of stainless steel with platinum, palladium and gold show that platinum has a dual effect on toe passivation of stainless steel. At

            first stainless steel goes into the zone of unstable passivity, since the overvoltage of           platinum for discharge of hydrogen ion is low; in the next stage stainless steel goes        into the zone of stable passivity, since oxygen in solution is reduced more easily on            platinum than on stainless steel. The efficiency of platinum is due to this dual activity,      not shared by palladium or gold. (23)

References

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22. Di Maggio, R., Fedrizzi, L., Rossi, S., Scardi, P.  (1996).  Dry and wet corrosion behaviour of AISI 304 stainless steel coated by sol-gel ZrO2—CeO2 films.  Thin    Solid Films, 286, 1-2, 127-135.  Retrieved October 22, 2008 from         ScienceDirect            database.

23. Bianchi, G., Barosi, A., Trasatti, S.,  (1965).  Anodic protection of stainless steel by    galvanic coupling with platinum.  Electrochimica Acta, 10, 1, 83-95.  Retrieved    October 21, 2008 from ScienceDirect database.

 

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Improving Corrosion Resistance of Stainless Steel. (2016, Oct 17). Retrieved from https://graduateway.com/improving-corrosion-resistance-of-stainless-steel/

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