Influence of the alloy element on corrosion morphology of the low alloy steels exposed to the atmospheric environments

Morphology of the corroded surface of low alloy steels beneath rust after long-term exposure test in the atmospheric environment was analyzed. The form of the corroded surface was measured with the laser displacement sensor scanning the surface. The resultant height map was divided by the mesh and the maximum corrosion depth was calculated in each cell. The maximum depth was arranged by the extreme value analysis. From this analysis two kinds of corrosion patterns were distinguished; i.e., uniform corrosion and local corrosion. Electrolytic iron shows the only uniform corrosion pattern. The addition of Cu, Ni and Cr changed the form of the corroded surface from the uniform corrosion to the combined pattern (uniform corrosion + local corrosion). The addition of Cr has a marked effect in changing the corrosion pattern.     

The pitting corrosion of low alloy and mild steels

Electrochemical investigations and microscopical examinations of pitting corrosion of low alloy (weathering) steels, copper bearing steel and plain carbon steel have been performed. Higher inclination to pit formation in steels containing alloying elements were ascertained in corrosive environments containing chlorides. Scanning electron microscopy enabled the determination of differences in morphology of pits formed on the steels under study.     

Effect on marine immersion corrosion of carbon content of low alloy steels

This paper attempts to reconcile information from a number of different sources about the effect of small changes in carbon content on the immersion corrosion of specimens of normal commercial mild and low alloy steels. It does so through interpreting the data reported in the literature in terms of the recently proposed theoretically based phenomenological model for marine immersion corrosion. This model postulates different corrosion phases as corrosion progresses. When the experimental results are interpreted in terms of the model it is found that carbon content has minimal effect on the kinetically controlled corrosion phase. The next phase, when corrosion rate is controlled by oxygen diffusion, is also unaffected, in agreement with theoretical predictions. However, carbon content does affect the two anaerobic phases, with increased corrosion as the carbon content and the water temperature increase. The model allows apparently conflicting observations to be reconciled and shows that carbon content may be influential for longer-term corrosion and for corrosion in tropical waters.     

Influence of carbon content and microstructure on corrosion behaviour of low alloy steels in a Cl containing environment

Corrosion behaviour of low alloy steels (A and B) with different carbon content was studied by a salt fog test and an outdoor test. A commercial weathering steel 09CuPCrNi was used for comparison. The corrosion resistance of steels A and B with homogeneous microstructures was better than that of the commercial weathering steel 09CuPCrNi in the salt fog test. Steel A with an ultra-low-carbon content had far less weathering resistance than the other steels in the outdoor test. Selective corrosion of large pearlite produces stress in initial corrosion product films. Uniform corrosion product films with few cracks tend to form on homogeneous microstructures such as ferrite and bainite, and this is advantageous for the formation of a compact rust layer in the initial stage of atmospheric corrosion. However, uniform microstructures will result in over even interfaces between rust layers and bases, which will lead to frequent peeling of rust layers from bases because stress is induced by large temperature fluctuations and wet-dry alternations. Protection of the rust layer on a low alloy steel is dependent on the rust density and the bonding performance of the rust-base rather than the proportion of the rust phase in the initial stage of atmospheric corrosion. These results indicate that homogenous microstructures, proper amounts of carbon content and fine carbon-rich phases that are produced by appropriate processes are beneficial for the corrosion resistance of steels.     

Effect of small compositional changes on marine immersion corrosion of low alloy steels

Small changes in the composition of mild and low alloy steels can effect their immersion corrosion behaviour. A number of comprehensive test programs for coupons immersed at different locations and recovered at different times have been reported. Comparison between them has also been attempted with modest success as well as leaving some apparent inconsistencies in the effect of some alloying elements.In this paper, a new comparative analysis of previously reported observations is reported. It employs a recently reported multi-phase phenomenological corrosion-time model, with different corrosion phases governing corrosion behaviour. Each phase is a function of time.The analysis shows that metal composition can influence the first, kinetically controlled corrosion phase and also the long-term anaerobic corrosion phases. However, during the phase controlled by oxygen diffusion through the corrosion product, metal composition is largely irrelevant, in agreement with theoretical predictions. It is shown that the several observations in the literature about the effect of particular alloys can be reconciled, including apparently conflicting observations about the effect of chromium content.     

The inhibition of stress corrosion cracking of stainless steels in chloride solutions

The inhibition of s.c.c. n AISI 316S stainless steel in 2N HCl by aromatic and heterocyclic substances has been studied. The evaluation of the inhibitors in reducing the s.c.c. of U-bend specimens has been carried out by micrographic measurements.The determination of anodic and cathooic polarization curves shows that the inhibiting action on s.c.c. is related to the effect of the organic compounds on the anodic dissolution process of the metal. An interpretation of s.c.c. in the chloride solutions is hypothesized involving an adsorbed layer of chloride ions. Tests performed with scratched electrodes seem to confirm the proposed interpretation.     

The effects of water pollution on the immersion corrosion of mild and low alloy steels

The short-term immersion corrosion of mild and low alloy steels in seawaters is known to be proportional to the concentration of dissolved oxygen (DO) in the bulk water. Longer-term corrosion is a function of the activity of sulphate-reducing bacteria and is influenced by the concentration of nutrients in the bulk water. These influences are examined in more detail for the corrosion of steels in the brackish waters of the River Thames and for several immersion corrosion sites on the Eastern Australian seaboard and in the North Sea. The published data sources were supplemented with plausible assumptions about environmental conditions. New interpretations of the data are provided based on the previously published model for immersion corrosion. For waters with negligible salinity and sulphate levels early corrosion loss was shown to depend on the dissolved oxygen content of the waters, and later corrosion loss was a direct function of nitrogenous nutrient (pollution) levels. This also applies to longer-term corrosion.     

Influence of external tensile stress on corrosion and trench formation of low alloy steel in a low H2S content sour corrosion environment

The influence of external tensile stress on corrosion and trench formation of low alloy steel in a low H₂S content sour corrosion environment was investigated. These experiments were conducted with steel for pipelines, and electrochemical methods were used. The results showed that external stress increases the amount of corrosion weight loss and trench depth by promoting the anodic dissolution reaction, and stress concentration was proven to be one of the driving forces for trench formation due to localized corrosion. Based on the experimental findings, the mechanism of trenching was discussed from the viewpoint of promotion of the anodic dissolution reaction by dislocations.     

The susceptibility to sulphide stress cracking of low alloy steels

The susceptibility to sulphide stress cracking (SSC) of low alloy Mn-V and Cr-Mo-Nb steels in standard NACE solution was determined. The critical stress (S_c) and threshold stress (σ_th) of steels were determined in both as-received and heat treated condition. In the as-received state the steel shows a highly susceptibility to SSC dominantly due to its microstucture. By quenching and tempering of steels at high temperature the SSC resistance was increased. It was found that the susceptibility to SSC for Cr-Mo-Nb steel is lower than that of Mn-V steel at a comparable yield strength. It was explained by the presence of different second phases in the ferrite matrix and spheroidized carbide particles resulting from the higher tempering temperature. In addition to Fe₃C, the presence of Cr₃C₂, Mo₂C and NbC in the ferrite matrix of Cr-Mo-Nb steel play an important role in their resistance to SSC. In all cases tested was S_c>σ_th which suggests of the importance of plastic deformation during the test. The microfractographic analysis of specimens before and after SSC testing have been done. Occurrence of cracks in the near outer surface of non-failed tensile specimens during 720 hours test suggested that SSC could be explained as a combination of hydrogen-induced cracking (HIC) and stress corrosion cracking (SCC).     

Effect of alloy element contents on caustic stress corrosion cracking of several stainless steels

The stress corrosion cracking behavior in caustic solutions (200 g/l sodium hydroxide, 10 g/l sodium chloride) of three austenitic (18Cr-10Ni-2.5Mo, 20Cr-25Ni-4.5Mo, 27Cr-31Ni-3.5Mo) and three duplex (23Cr-4Ni, 22Cr-5Ni-3Mo, 25Cr-7Ni-4Mo-N) stainless steels was examined. U-bend and Slow Strain Rate (SSR) tests were performed at 200–250°C. The negative influence of nickel in the lower range content for the 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo has been shown; when the nickel content is significantly increased (>30%), as in the case of the steel 27Cr-31Ni-3.5Mo, an increase of SCC resistance has been detected. The negative effect of molybdenum, mainly on the behaviour of duplex stainless steels, has also been evidenced. The duplex stainless steels show better caustic SCC resistance than austenitic stainless steels type 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo. The best behaviour has been found for the less-alloyed steel 23Cr-4Ni.     

Influence of tensile straining on the permeation of hydrogen in low alloy Cr-Mo steels

The effect of dynamic tensile straining on hydrogen permeation through low alloy Cr-Mo steel samples was studied with the electrochemical permeation technique. The hydrogen steady-state permeation current through large flat tensile specimens mounted between the two compartments of an electrochemical permeation cell undergoes different types of changes, depending on the charging conditions, on the steel’s composition and microstructure and on the strain rate. Dynamic trapping of hydrogen to strain-induced dislocations which leads to a deviation of the permeation current below the initial steady-state value is mostly observed when the external hydrogen activity and the strain rate are large. However, the hydrogen permeation current through Cr-Mo steels with a bainitic microstructure appears to be less sensitive to tensile straining up to large deformation levels than a lower alloyed Cr-Mo steel with a ferrito-pearlitic microstructure. On the other hand, the enhancement of the steady-state hydrogen permeation current observed during tensile straining if specific experimental conditions are met strongly suggests a mechanism of hydrogen transport by mobile dislocations which contributes to hydrogen permeation.     

Influence of steel gauge on the microstructure and corrosion performance of zinc alloy coated steels

Zn-4.8 wt% Al coated steels exhibited a polynomial increase in Zn loss from the cut edge with increasing steel gauge for weight loss and SVET Zn loss in 0.1% NaCl in contrast to a linear increase observed for hot dipped galvanised (HDG) steels. Increasing steel gauge decreased nucleation and increased growth of Zn dendrites for Zn-4.8 wt% Al coated steels. SVET data showed larger Zn dendrites associated with thicker gauges produced more damaging anodes than small dendrites on thinner samples. A polynomial relationship was observed between dendrite size and Zn loss suggesting that microstructure influenced Zn loss in unison with the increased cathodic activity on thicker steels.     

Influence of cavitation on the passive behaviour of duplex stainless steels in aqueous LiBr solutions

The objective of this work is to study the influence of cavitation on the passive behaviour of EN 1.4462, its filler metal (EN 1.4462F), and the welded metal (EN 1.4462W) obtained by Gas Tungsten Arc Welding using electrochemical techniques. The hydrodynamic conditions of the medium were modified using an ultrasonic-induced cavitation facility.Potentiostatic experiments were used to study the effects of cavitation on the passive behaviour of the alloys. The experiments were carried out in 850 g/L LiBr solutions with and without an inhibitor (Lithium Chromate). The solution with Li₂CrO₄ (commercial solution) contains LiOH as the pH regulator. The potentiodynamic cyclic curves of the stainless steels under the static condition were used to set the values of the imposed potentials.In this work, the electrochemical behaviour of the alloys described by the potentiodynamic curves has been related to their passive behaviour under potentiostatic conditions when the pulses of cavitation were applied. The results demonstrate that cavitation affects the passive behaviour of the alloys; the influence depends on the potential applied and on the presence or absence of chromates in the medium. Only under certain circumstances the hydrodynamic conditions can suppose a breakdown of passive film formed under static conditions.     

Influence of a prefilming treatment on the corrosion resistance of aluminium brass in aqueous solutions

The influence of a short-time prefilming treatment with benzimidazole-2-thiol on the corrosion resistance of Cu Zn 22Al 2 has been examined. The efficiency of the prefilming treatment has been evaluated by recording the anodic and the cathodic polarization curves in 0.1 N NaCl. Ellipsometric measurements have been carried out in order to obtain some information on the properties of the layer. Comparative data on “prefilmed” and “bare” Cu Zn 22Al2 specimens have been obtained in aqueous chloride solutions circulating in a corrosion loop and by tests in moist atmosphere containing SO₂. Galvanic corrosion testing have been carried out by using small not prefilmed areas and large prefilmed ones, simulating in this way the partial removal of the film from the metallic surface. The results suggest that a metal-inhibitor film forms on Al-brass, which mainly affects the cathodic reaction rate. Furthermore, the tests clearly indicate that the effect of benzimidazole-2-thiol is strongly influenced by a pre-existing oxide layer on the alloy surface. A good protection against atmospheric corrosion is reached by prefilming treatment. Corrosion loop testing indicate that prefilming does not hinder the formation of protective corrosion products on the surface of the alloy in flowing aqueous solutions.     

The stress corrosion cracking behavior of austenitic stainless steels in boiling magnesium chloride solutions

The change in the mechanism of stress corrosion cracking with test temperature for Type 304, 310 and 316 austenitic stainless steels was investigated in boiling saturated magnesium chloride solutions using a constant load method. Three parameters (time to failure; t_f, steady-state elongation rate; l_ss and transition time at which a linear increase in elongation starts to deviate; t_ss) obtained from the corrosion elongation curve showed clearly three regions; stress-dominated, stress corrosion cracking-dominated and corrosion-dominated regions. In the stress corrosion cracking-dominated region the fracture mode of type 304 and 316 steels was transgranular at higher temperatures of 416 and 428 K, respectively, but was intergranular at a lower temperature of 408 K. Type 310 steel showed no intergranular fracture but only transgranular fracture. The relationship between log l_ss and log t_f for three steels became good straight lines irrespective of applied stress. The slope depended upon fracture mode; −2 for transgranular mode and −1 for intergranular mode. On the basis of the results obtained, it was estimated that intergranular cracking was resulted from hydrogen embrittlement due to strain-induced formation of martensite along the grain boundaries, while transgranular cracking took place by propagating cracks nucleated at slip steps by dissolution.     

A comparison of potentiodynamic,current decay and straining electrode experiments in assessing the stress corrosion cracking susceptibilities of low alloy ferritic steels

A series of ferritic steels containing chromium, molybdenum, nickel and titanium additions and having different propensities towards stress corrosion cracking in 1N Na₂CO₃ + 1N NaHCO₃ solutions have been used to compare potentiodynamic, current decay and straining electrode measurements as means of predicting the cracking susceptibilities. Potentiodynamic polarization curves probably offer the simplest means of defining the potential range for cracking, current decay measurements provide data that give the closest fit to the observed crack velocities, whilst the straining electrode experiments most clearly define difference in the details of the mechanisms of cracking in the different steels.     

Pitting and crevice corrosion behaviour of high alloy stainless steels in chloride‐fluoride solutions

The localised corrosion resistance (pitting and crevice corrosion) of two high alloy stainless steels, namely superduplex (SD) and superaustenitic (SA), has been studied in chloride‐fluoride solutions at pH values ranging from 2 to 6.5. The pitting potential (E_pit) and crevice potential (E_cre) have been calculated for these test media using electrochemical techniques (continuous current). The Critical Pitting Temperature (CPT) and Critical Crevice Temperature (CCT) are in both materials lower then the room temperature. In spite of this fact and due to the high repassivation rate, the resistance of these materials to localised corrosion is high in the tested media. At the highest tested concentration of aggressive anions and pH 6.5 both materials undergo a generalised attack.