Materials selection in the oil and gas industry relies on engineering standards, such as NACE TM0177 and NACE TM0284, which stipulate that oxygen contamination should be avoided during materials testing in H 2 S-containing media. In this second paper, as part of a series of articles that evaluates how traces of oxygen modify the corrosion of pure iron and hydrogen permeation across iron membranes in H 2 S-containing solutions, the impact of changing the H 2 S partial pressure from 100 kPa to 0.1 kPa was investigated. It was found that bulk solution chemistry for all H 2 S partial pressures changes with time, due to the formation of H 2 S–O 2 reaction products (sulfates, sulfites, and thiosulfates), which results in bulk solution acidification. Electrochemical and weight-loss measurements confirm that Fe corrosion rates in baseline well-deaerated H 2 S-containing solutions decrease with decreasing H 2 S partial pressure, although these are observed to be much higher under continuous oxygen contamination. With decreasing H 2 S partial pressure, hydrogen uptake in Fe also decreases, due to lower and lower concentrations of dissolved H 2 S and the associated increase in pH. However, even at 1 kPa and 0.1 kPa H 2 S, permeation effciencies remain close to 100% when no O 2 contamination is present. The hydrogen uptake is always relatively lower in Fe exposed to oxygen-polluted H 2 S solutions. Permeation efficiencies decrease continuously. From electrochemical data and surface characterization, these observations at lower H 2 S partial pressures are attributed to the disruptive effect of oxygen on the nature of sulfide corrosion products, and hydrogen entry promotion, along with the contribution of an additional cathodic reaction that does not result in hydrogen entry into the metal

Materials selection in the oil and gas industry relies on engineering standards, such as NACE TM0177 and NACE TM0284, which stipulate that oxygen pollution should be avoided during materials testing in H2S-containing media. In this paper, we explore the manner in which traces of oxygen can modify the test solution chemistry and the corrosion of/hydrogen permeation across iron membranes in H2S-containing solutions. Oxygen pollution is shown to strongly influence solution chemistry, through the introduction of sulfur-oxygen reaction products resulting in bulk acidification. Weight loss, electrochemical methods, and solution chemistry measurements conclude that iron corrosion rates in the presence of oxygen pollution are doubled, when compared against the control system (without oxygen pollution). Unexpectedly, despite a lower pH and higher corrosion rates in the oxygen-polluted H2S-containing solutions, the hydrogen permeation rate decreases monotonically, relative to the control. We discuss how this observation is most likely related to a disruption of sulfur adsorbates involved in hydrogen entry promotion. 

In this study, the cathodic activity of biofilmed stainless steel surfaces was investigated at two exposure depths at the same location at 1,020 m and 2,020 m depth. For this purpose, a set of passive materials and sensors were exposed for 11 months in Azores, in the Atlantic Ocean. Characteristic cathodic depolarizations due to biological activity were observed in intermediary and deep water. However, a strong cathodic activity was only measured in deep water. Potential ennoblement appeared between 80 d and 200 d, depending on the exposure depth and the experimental setup used. In a given environment, the biological cathodic activity appears to be strongly related to the limiting parameter of the reaction, which can be anodic or cathodic. The biofilm sensors exposed for the first time in open, deep water appear relevant to discriminate cathodically “strongly-active” and “weakly-active” biological activity. Under cathodic control, a high current density was measured on stainless steel in deep seawater. The experimental setup used is particularly relevant as it allows determination in situ of the maximal cathodic current density.

The reactivity of the α-phase of Al-Zn (Zn-68 wt% Al, Al 5.2 Zn) in deaerated 0.1 M NaOH solution (simulating industrial pretreatments) was investigated and compared with that of pure Al and Zn. The elementary phenomena of metal oxidation, dissolution, oxide formation, and hydrogen evolution were decoupled using atomic emission spectroelectrochemistry. At the open-circuit potential, the Al 5.2 Zn phase reacted similarly as pure Al, undergoing selective Al dissolution to form a Zn(0) enriched layer. The Zn in the alloy shifted the potential to just below the onset of Zn dissolution. Elementary polarization curves showed that Zn dissolution was similar for the Al 5.2 Zn phase as for pure Zn. Near the open-circuit potential, Zn dissolution was faradaic limited by the formation of surface Zn(OH) 2 . At higher temperature, significant amounts of ZnO formed resulting in passivation. For the Al 5.2 Zn phase, the rates of Al and Zn dissolution were determined by a charge transfer mechanism across the ZnO film. Kinetic parameters (activation energies and Tafel slopes) were measured for some of the elementary processes. 

The corrosion risk for stainless steel components is not the same in all seawaters, with more failures generally reported in tropical seas. In this study, the influence of biofilm on electrochemical behavior and corrosion resistance of passive films of high-grade alloys was studied in different seawaters, including temperate seawater (France-Brest, North Atlantic Ocean), tropical seawater (Malaysia-Kelatan, Meridional China Sea), and intermediate conditions in terms of temperature (Brazil-Arraial do Cabo, South Atlantic Ocean). The stabilized open-circuit potentials and the polarization behavior of high-grade stainless steels were measured as a function of temperature in all of the tested field marine stations, providing quantified data and direct comparison of the biofilm-enhanced corrosion risks. Significant differences were measured in tropical and in temperate seawaters in heated conditions. Above 37°C, the biofilm activity was much more pronounced in tropical seawater compared to Atlantic Ocean sites, leading to much higher localized corrosion risk. Crevice corrosion of eight high-grades passive alloys was also studied with the use of crevice formers specifically developed for tube geometries. Duplex UNS S32205, superduplex UNS S32750, hyperduplex UNS S33207 and S32707, and 6Mo stainless steels UNS S31266 have been evaluated together with Ni-based alloys UNS N06845 and N06625. In the more severe conditions, the high-grade alloys UNS S32707 and the 6%Mo UNS S31266, both with pitting resistant equivalent number (PREN) around 50, showed better performance than commonly used superduplex UNS S32750 and UNS S39274 (PREN 40). The corrosion results are discussed regarding the monitored biofilm-induced depolarization measured in the different test conditions.

Chlorination is widely used in seawater systems to avoid fouling and associated microbial-induced corrosion. Free chlorine is a strong oxidizing agent that prevents biofilm formation on immersed surfaces when used above a certain content. However, the presence of residual chlorine associated with the relatively high chloride content in seawater significantly increases the risk of localized corrosion for most stainless steels. In the present study, a module initially developed to quantify the formation of electroactive biofilms on stainless steels has been used to assess the corrosiveness of chlorinated seawater. Both the electrochemical potential and the cathodic current were measured on super-duplex stainless steel as a function of residual chlorine levels and seawater temperatures. In parallel, long-term localized corrosion tests have been performed in simulated environments to assess the environmental limits for the safe use of high-grade stainless steels in chlorinated seawater. It includes crevice corrosion exposure tests using adapted ISO 18070:2015 crevice formers and internal tube pitting corrosion exposure tests in model tube heat exchangers simulating heat flux from 35°C to 170°C. The synergetic effect of residual chlorine content and temperature on the risk of localized corrosion has been quantified. Corrosion resistance properties are correlated to the electrochemical monitoring data, and the environmental limits of selected base materials stainless steels have been established for duplex stainless steel UNS S32205, super-duplex stainless steel UNS S32750, hyper-duplex stainless steels UNS S32707 and UNS S33207, and the high-grade austenitic stainless steel UNS S31266.

Laboratory tests have been performed to determine how climatic parameters, e.g., relative humidity, temperature, and the amount of sodium chloride (NaCl), influence the corrosion rate of magnesium alloys AZ91D (UNS M 11916) and AM50 (UNS M 10500). The effect of the surface state also has been investigated. The exposures were performed at 75%. 85%. and 95% relative humidity (RH) and at 25°C and 35°C. The amount of NaCl ranged from 14 μg cm-2 to 240 μg cm-2 . The corrosion rate of both materials increased as a function of temperature, RH, and amount of NaCl. A strong influence of the surface state, i.e., as-cast or polished, was observed mainly due to the combined effect of an active surface layer and the roughness of as-cast surfaces. © 2004, NACE International.

The effects of temperature, relative humidity, and wet-dry transition on initiation and propagation of filiform, corrosion on coated aluminum alloy 6016 (AA6016 [UNS A96016]) have been studied. Corrosion products formed in the filament were analyzed using Fourier transform infrared (FTIR)- microspectroscopy. The aluminum surfaces were tested in both milled finish and grinded conditions with chromium, phosphate, and titanium-zirconium-based pretreatment. An electrodeposited coat (ED coat) and a full paint system used for automotive applications (ED coat + topcoat) were investigated for the different combinations of mechanical finish, surface pretreatment, and coating system. In the temperature range between 5°C and 50°C, filiform corrosion, or underfilm corrosion in general, increased significantly. The effect of relative humidity and wet-dry cycling, on the other hand, seems to be strongly influenced by parameters like pretreatment, coating system, and temperature. Filiform corrosion was the highest in the range from 75% to 95% relative humidity (RH), and a distinct maximum was observed at 85% RH for some coating systems. However, filiform. corrosion propagated at humidity down to 40% to 50% RH. For specimens with chromate- and phosphate-based surface pretreatments, filiform, corrosion was lower after exposure to tests with wet-dry cycles. The samples with titanium-zirconium-based pretreatments, on the other hand, had a very poor filiform corrosion resistance in the cyclic test compared to exposures at constant relative humidity. © 2004, NACE International.

In this study, the effect of test conditions on the formation of selective dissolution during sour testing was investigated on a cold-rolled duplex stainless steel UNS S32750. All experiments were conducted in NaCl 150 g/L and pH2S = 0.3 bar. Different pHs between 3.3 and 4.5 were studied at 80°C. Based on tests performed under varying conditions, it is demonstrated that selective dissolution (SD) competes with cracking and that under conditions leading to the formation of a large area with SD, the presence of this type of corrosion can hide the susceptibility of the material to cracking. The presence of only SD after testing must therefore be considered with caution. SD initiated also without applied stress showing that the phenomenon is correlated to a loss of passivity. From electron backscattered diffraction (EBSD) analyses and electrochemical monitoring the formation of SD under the test conditions considered in this work is correlated to the instability of the passive film and not to any superficial singularities of the material or specific crystallographic orientations. 

The scanning Kelvin probe (SKP) is a non-destructive technique for measuring the surface distribution of the Volta potential with a high spatial resolution of a few tens of micrometers. The SKP technique allows in situ studies of the localized corrosion processes under atmospheric weathering conditions, on metal surfaces, or underneath organic coatings. In the present study, the SKP technique was used to follow the kinetics of underpaint corrosion from a defect applied on steel coated with thick marine paint systems (0.4 mm to 0.5 mm) as a function of exposure time in an accelerated corrosion test. Three different paint systems were investigated. In addition, the influence of surface cleanliness in terms of salt concentration on a steel substrate prior to paint application was investigated using the SKP technique. The results showed the high efficiency of the SKP technique for early corrosion evaluation under thick paints on steel substrate.

Atmospheric corrosion of Zn and Zn alloys with local NaCl contaminations was studied in situ by scanning Kelvin probe. The corrosion process was accompanied by the formation of clearly separated anodic and cathodic locations and local galvanic cells on the surface. The corrosion activity correlated to variations in Volta potential during air to nitrogen transients. The effect of dielectric and semiconducting solids on the efficiency of cathodic reaction on zinc was investigated. The rate of oxygen reduction was high on semiconducting ZnO films with defective structure, as they are effective donors of electrons. Mg, Ti, Cr, and Al were alloyed to zinc to modify the composition of surface oxide films. The addition of Mg provided the most effective corrosion inhibition, blocking completely the spreading of the cathodic area from NaCl contamination. The other elements had minor influence, but might be applied for further improvement of the Zn-Mg system. The effect of Mg is believed to be connected to semiconducting properties of the formed surface oxide film.

Aluminum alloys are not immune to corrosion which can take the form of localized corrosion. Thus, the assessment of the corrosion behavior of aluminum alloys under atmospheric conditions is a major topic for the aerospace industry. One major difficulty in this task is the lack of robust and reliable accelerated corrosion test(s) in this field. Indeed, several tests as the neutral salt spray test (ASTM B117) are used to assess the general corrosion resistance of aluminum, but these tests were not developed specifically for the aerospace industry and are not representative of service conditions. The aim of the present study was to compare the results of various accelerated corrosion test conditions (ASTM B117, VDA 233-102, Volvo STD 423-0014) with newly developed test conditions. Hence, different accelerated corrosion tests were designed by varying several parameters in the Volvo STD 423-0014 such as the salt concentration, time of wetness, and relative humidity. The results obtained on eight aluminum alloys (2xxx, 7xxx, and Al-Li alloys) were then compared to marine exposures. From the results, one test provides the same type of corrosion attacks on the different alloys under atmospheric exposures in the marine site and a good acceleration factor.

Several cases of ceiling collapses and other failed elements have been reported in indoor swimming pool halls in the last two decades. The collapses were caused by stress corrosion cracking (SCC) of stainless steel fastening elements covered with chloride deposits at temperatures as low as room temperature. The goal of this study was to assess the application limits of different austenitic and austenitic-ferritic (duplex) stainless steels subject to tensile stress and contaminated with chloride deposits in atmospheric non-washing conditions as a function of temperature (20°C to 50°C), relative humidity (15% to 70% RH), and deposit composition. Austenitic stainless steels Type 304 (UNS S30400) and Type 316L (UNS S31603) were susceptible to SCC in the presence of magnesium and calcium chlorides at temperatures of 30°C and higher and at low relative humidity. The tendency to SCC increased with increasing temperature and decreasing relative humidity. The corrosivity of chloride deposits under given exposure conditions decreased in thefollowing order: calcium chloride (CaCl2) > magnesium chloride (MgCl2) > sodium chloride (NaCl). It was governed by the equilibrium chloride concentration in the surface electrolyte formed as a result of interaction of a given salt with water vapor in the air. Threshold values of the minimum chloride concentration and relative humidity intervals leading to SCC were established for Type 304 and Type 316L. Duplex stainless steels S32101 (UNS S32101), 2304 (UNS S32304), 2205 (UNS S32205), and 2507 (UNS S32750) were resistant to SCC but corroded selectively with the maximum depth of 200 μm. Austenitic stainless steels Type 904L (UNS N08904) and Type S31254 (UNS S31254) showed no tendency to SCC.

Corrosion performance of austenitic-ferritic (duplex) stainless steels UNS S32101, S32202, S32304, and S32205 and austenitic stainless steels UNS S30403 and S31603 was studied in the presence of chloride deposits simulating non-rinsing atmospheric conditions. The effect of temperature, relative humidity, concentration, and composition of the chloride deposits on the tendency for atmospheric, low-temperature, chloride-induced stress corrosion cracking (SCC), pitting, and selective corrosion was assessed using prestressed samples with a circular weld. In the presence of calcium chloride, SCC was observed at temperatures as low as 20°C and 30°C on the austenitic stainless steels UNS S30403 (Type 304L) and UNS S31603 (Type 316L), respectively. The only cases of SCC of tested duplex stainless steel grades were found at 70°C, which otherwise suffered mainly selective dissolution of the ferrite phase with one order lower depth of attack. The initiation of SCC and selective/pitting corrosion was governed by the equilibrium chloride concentration in a solution formed by contact with chloride-containing deposits and with air at a given relative humidity. Threshold levels of critical chloride concentrations, critical relative humidity in the presence of specific deposits, and maximum temperatures for safe applications of the studied grades were established.

The ability of chromate as an anticorrosive pigment incorporated into primer coatings to inhibit the corrosion of galvanized steel has been studied using cell modeling conditions at defects of painted sheets. The experiments were performed at different temperatures (4, 22, and 40° C) and chloride concentrations (1 mmol/L and 10 mmol/L) with coatings releasing from 20 mg/m2 to 100 mg/m2 chromate per 48 h. After 1 h to 48 h of exposure, the solutions were analyzed by ion chromatography (IC) and the metal samples were studied using Fourier transformed infrared (FTIR), energy-dispersive x-ray (EDX), and x-ray absorption near-edge structure (XANES) spectroscopy. The XANES measurements detected the presence of a Cr(III)/Cr(VI) layer containing approximately 15% of Cr(VI) on the initially bare zinc surface. Corrosion of the samples exposed with the chromated primers was clearly inhibited in comparison with blank samples, with an inhibition efficiency ranging from 50% to 95% according to the exposure conditions. Thus, the results showed that chromate was transported from the pigmented primer to the bare metal surface and formed a protective layer on it. Moreover, an enrichment of the pit centers with chromium wasfound with EDX, suggesting the ability of chromate to seek the active, or formerly active, corrosion sites and preferentially adsorb at these locations. The extent of corrosion deterioration of the samples and chromate consumption was regarded with respect to temperature, chloride concentration, total amount of released chromate, and the kinetics of the release. The developed experimental technique was tested on a vanadate system as well. The experimental setup proved to be a simple and efficient tool for future testing of paints, inhibitors, materials, or aggressiveness of environment, as well as for fundamental studies. © 2004, NACE International.

The formation and the corrosion protection of newly formed chromium-rich layers on bare zinc surfaces were studied to model the conditions in defected areas of both organic and conversion chromate coatings that are in contact with water environments contaminated with different amounts of chloride ions. Composition of the layers was identified with Fourier transformed infrared spectroscopy (FTIR), x-ray absorption near-edge structure (XANES), and secondary ion mass spectroscopy (SIMS). The presence of chloride in the range from 0.06 mM to 1,000 mM in the chromate treating solution had almost no effect on the amount of chromate adsorbed on zinc. Three independent technique showed that a more than 4-order increase in chloride concentration results in the drop of the chromate content in the surface film only by 20% to 25%. Cr(VI)-to-total Cr surface ratio was close to 0.3 and constant under present experimental conditions. More chromium was detected in the outer region of the film, whereas chloride accumulated in the inner region. As a result of the linear increase of the surface chloride concentration with the chloride concentration in the chromate treating solution, the chloride-to-chromate surface molar ratio increased sharply. The rate of reduction of Cr(VI) to Cr(III) and the corrosion rate of zinc exposed to atmospheric weathering conditions increased significantly with the chloride-to-chromate ratio. The chromate coatings showed good stability and a high level of corrosion protection, up to the ratio of approximately 2. It represented a threshold value below which relatively low rates of the chromate reduction and zinc corrosion were observed, since the significant part of the chloride ions was inactivated in the first hours of exposure by the formation of insoluble corrosion products. A negative effect of the increasing chloride-to-chromate surface molar ratio on corrosion can be seen in the increasing ability to reduce oxygen on the zinc surface measured by the scanning Kelvin probe (SKP) technique. Inhibition of the cathodic reaction by chromate was less effective at higher ratios.

The open-circuit potential is one of the main driving forces of galvanic corrosion when two dissimilar metals are in electrical continuity in an electrolyte. From the existing literature, the galvanic series which provides averaged potential of metallic materials in seawater is generally restricted to ambient/ standard conditions or to a limited number of alloys. However, advanced materials have been developed in the last decade and the corrosion potential of any alloy immersed in seawater may be strongly affected by environmental factors. There is a lack of information on these purposes (e.g., effect of dissolved oxygen content, temperatures, chlorination at different levels, or recently developed alloys, etc.). In this work, the open-circuit potential of different stainless steel grades, as well as nickel-based and copper-based alloys, has been systematically measured in seawater under different experimental conditions. In particular, the effect of temperature (from 30°C to 70°C), oxygen content (from 10 ppb to saturation), and chlorination level (from 0 ppm to 0.5 ppm) have been studied. The work can also be used for material selection in terms of risk of bi-metallic corrosion when coupling two materials under these conditions.