Corrosion Engineering,Science and Technology: The International Journal of Corrosion Processes and Corrosion Control

Active screen plasma is a recently developed plasma surface alloying technique, which has shown potential for addressing some drawbacks associated with conventional direct current plasma processes. In this study, the corrosion performance of untreated, direct current and active screen plasma carburised AISI 316 was investigated by immersion in a boiling solution of sulphuric acid. The experimental results show that the corrosion behaviour of expanded austenite produced by low temperature plasma carburising is controlled by the type and density of surface defects; the corrosion properties of the active screen plasma carburised material are superior to that produced by direct current plasma because of the significantly reduced edge effect and surface defects; and the bias level used in the active screen carburising treatment has a profound effect on the corrosion performance of the material. Based on the experimental results, the corrosion mechanisms involved are discussed.

The microarc oxidation (MAO) of AM60 magnesium alloy in two kinds of electrolytes is studied in this paper to compare the influence of electrolytes on the properties of the MAO coatings. One concentrated electrolyte is composed of 100 g L⁻¹ NaOH+20 g L⁻¹ Na₂B₄O₇.10H₂O+50 g L⁻¹ C₆H₅Na₃O₇.2H₂O+60 g L⁻¹ Na₂SiO₃.9H₂O and the other is a dilute solution, which is composed of 5 g L⁻¹ NaOH+15 g L⁻¹ Na₂SiO₃.9H₂O. It was found that the sparking behaviour in dilute electrolyte is more intensive and the breakdown voltage U _B is higher (the U _B in dilute electrolyte is 282 V, while it is 82 V in the concentrated electrolyte). The thermal effect during the MAO in the two electrolytes is also measured, and the temperature increment in dilute electrolyte is more than twice of that in the concentrated electrolyte. The corrosion resistance of the MAO films is studied by electrochemical impedance spectroscopy and potentiodynamic polarisation techniques and the microstructure of the films is examined by SEM. The electrochemical tests show that the MAO film formed in the concentrated electrolyte is more corrosion resistant. Scanning electron microscope shows that the microstructure of the MAO film in dilute electrolyte is porous and thin, which accounts for its poor corrosion resistance. The experiment results show that high U _B does not mean good corrosion resistance.

The weld metal corrodes much faster than the parent metal if type 316L steel welded with a matching filler is exposed to hot organic acids. Nine commercial filler metals were evaluated by preparing welded coupons and exposing them to different plant environments (with different organic acid mixtures and temperatures) for several months. The filler metals were chosen to yield different solidification modes, and to vary chromium, molybdenum and nickel contents. Only filler metals that are much more highly alloyed than the parent metal gave lower corrosion rates than the parent metal. Partitioning during and after solidification, and the average alloy content, both affect weld bead corrosion.

Stress corrosion cracking tests (SCC) were performed on 1018 low carbon steel into 0·5M NaCl solution at different pH values and temperatures of 25, 50 and 70°C using the slow strain rate testing technique. Evaluations were complemented with hydrogen permeation measurements and electrochemical impedance spectroscopy. Studies show hydrogen diffusion effects depending on temperature and pH. The maximum hydrogen diffusion was observed at 70°C (pH<1). The SCC susceptibility was measured as the percentage reduction in area and the time to failure and was maximal at pH<1 at the temperatures studied. The most likely mechanism for the cracking susceptibility of 1018 steel in the chloride solutions seems to be hydrogen assisted anodic dissolution assisted by hydrogen embrittlement. After failure, the fracture surfaces were observed by scanning electron microscopy and chemical analysis was obtained by X‐ray energy dispersive spectroscopy. The specimens tested in air exhibited a ductile type of failure, whereas, in the corrosive solution the specimens showed a brittle fracture.

The effect of compressive loading on stress corrosion cracking of aluminium alloy 7075(W) was studied using specimens containing simulated fastener holes and the results were compared with the behaviour of double cantilever beam specimens with tensile loading. Crack lengths were measured with an eddy current bore probe and confirmed by optical metallography. Intergranular stress corrosion was observed in both tensile and compressive loading. Cracks initiated readily in 3·5%NaCl solution with tensile loading and the alloy was much less susceptible in compression, with cracking being detected only after 14 day exposure to EXCO solution. No cracking was observed in unstressed specimens, even after 56 day continuous exposure to EXCO solution. In both cases, SCC was explained by an active path mechanism due to the galvanic interaction between grain boundary precipitates and adjacent precipitate free zones. Compressive loading produced a positive strain at the crack tip, which ruptured the protective corrosion product film, maintaining the path of preferential corrosion.

The presence of organic acid species in formation water of oil and gas reservoirs has long been suspected to contribute to the corrosivity of CO₂ corrosion. The effect had been regarded as secondary until new findings in the 1980s and 1990s revealed the increase in corrosion rate in CO₂ environment with the presence of acetic acid species. This is potentially detrimental as most of the predictive models used in corrosivity assessment of the field do not incorporate considerations of the effects of acetic acid species. This paper presents experimental corrosion rates of carbon steel in CO₂ environments with the presence of low concentrations of acetic acid species under the turbulent flow conditions, based on linear polarisation resistance analysis. The experimental corrosion rates are compared with the corrosion rates predicted by industrial predictive models such as Norsok and Cassandra. Based on the findings, the authors can conclude that acetic acid species increase the corrosion rate of carbon steel in CO₂ corrosion and standard predictive models do not capture this effect. An empirical equation is proposed to cater for the presence of low concentrations of acetic acid in CO₂ corrosion under turbulent conditions.

Electrochemical impedance spectroscopy and potentiodynamic polarisation measurements were used to verify the effects of temperature and acetic acid (HAc) on the anodic and cathodic reactions in CO₂ corrosion of 316L stainless steel in 3·5% NaCl solution. The tests were carried at 30, 60 and 90°C. The initiation concentration of HAc was varied between 0 and 3000 ppm. After the potentiodynamic polarisation measurements, the surface morphologies for every condition were detected by using scanning electron microscopy. In the present work, the fitting parameters of polarisation curves, such as the corrosion potential E _corr, the corrosion current density I _corr and Tafel shopes (b _a and b _c), were mainly discussed. In addition, the equivalent circuit was used to fit each Nyquist plot in order to obtain the important parameters. In the last section, the probable pitting corrosion mechanisms of pit initiation and growth were also analysed and validated. The results indicated that temperature and the addition of HAc have an important synergistic effect on the pitting corrosion behaviour and mechanism, leading to the increased corrosion rate with temperature and HAc.

Steel in the galvannealed condition is preferred over simple galvanised steel for autobody components due to its superior corrosion resistance and relatively easy weldability. However, during resistance seam welding, the area at the weld nugget completely loses its zinc coating due to melting. On the other hand, the zinc coating in the heat affected zone (HAZ) is not totally removed although it is affected by the heat input consequent from the welding process. Although this is potentially a problem, the weld nugget has generally less probability of exposure to the open atmosphere as it is usually enveloped within the HAZ. In the present investigation, a reduction in zinc coating thickness in the HAZ from 9 to 1·5 μm has been observed with increasing resistance weld heat input from 775 to 1310 kJ. This reduction is responsible for increasing the corrosion rate from ∼1 to ∼5 mpy. Therefore, in order to reduce corrosion at the HAZ, the weld heat input has to be restricted.

Compositionally modulated multilayer coatings of Ni/Zn, Ni/Zn–Fe/Zn and Ni/Zn/Zn–Fe, for corrosion protection performance, were deposited on steel substrates using a dual bath technique. The coating protection performance was evaluated using Tafel extrapolation, anodic polarisation and salt fog corrosion tests. The performance of Ni/Zn–Fe/Zn and Ni/Zn/Zn–Fe multilayers was found to be better than that of Ni/Zn multilayer. Considering the corrosion mechanism, this can be attributed to an improvement in the corrosion resistance of the sacrificial layers by substitution of zinc within the Zn/Zn–Fe or Zn–Fe/Zn layers.

In indoor swimming pools stainless steels are used not only due to structural and economic reasons but also due to aesthetic considerations. And therefore not only safety relevant load bearing components have to fulfil their tasks but also the visual appearance should be free of any staining. In a lot of cases basic requirements (e.g. design criteria, material selection and maintenance) being essential parts of technical regulations were not taken into account. Typical case studies with explanation of the cause of the corrosion problems and possibilities to prevent them are shown in the present paper.