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