In situ AFM detection of pit onset location on a 304L stainless steel

A set-up combining an AFM and an electrochemical cell has been used to study in situ the local corrosion of a 304L stainless steel in an aqueous chloride-containing solution. The focus was made on the sites where pits were initiated under controlled potential in order to establish whether or not the pits were randomly distributed at the nanoscale. Grain boundaries and surface stoichiometric inhomogeneities appeared to concentrate pit onsets significantly. The influence of the mechanical history of the material, especially soft surface strain hardening, on the location of the first pits has been shown. The study revealed that 70% of the pits initiated at strain hardened areas resulting from mechanical polishing. A plausible model has been suggested to explain such a behaviour.     

An experimental study of crevice corrosion behaviour of 316L stainless steel in artificial seawater

The effects of applied torque on corrosion behaviour of 316L stainless steel with crevices were investigated using the cyclic potentiodynamic polarization method. Three kinds of crevices (316L-to-polytetrafluoroethylene, 316L-to-fluoroelastomeric and 316L-to-316L) were tested in artificial seawater at 50 °C. Corroded surface morphology was also investigated using scanning electron microscopy. Results indicate similar trends in crevice corrosion susceptibility with increasing applied torque. Among the three crevices, the 316L stainless steel specimen, coupled to the 316L stainless steel crevice former, is the most susceptible to crevice corrosion.     

Eradication of the corrosion-causing bacterial strains Desulfovibrio vulgaris and Desulfovibrio desulfuricans in planktonic and biofilm form using photodisinfection

Oil and gas pipelines often fail prematurely due to “microbiologically-influenced corrosion” (MIC). This occurs when free-floating bacteria collect on the inner pipeline surface, eventually forming complex adherent biofilms. Photodisinfection is an effective antimicrobial approach for several biomedical applications. This study evaluated the antibacterial efficacy of photodisinfection against two sulfate-reducing bacterial strains implicated in the process of MIC. Results showed that treatment reduced planktonic bacterial viability by >99.99%. Treatment of biofilms reduced viability by 99.9%, which was greater than the antibacterial effect observed using the biocide benzalkonium chloride under similar exposure parameters. These results suggest that photodisinfection may be useful in addressing MIC in industrial pipelines.     

Combined effect of resistance spot welding and post-welding sensitization on the degree of sensitization of AISI 304 stainless steel

This work studies the combined effect of resistance spot welding (RSW) and post-welding sensitization on the degree of sensitization (DOS) of AISI 304 stainless steel (SS) by using EPR and DLEPR tests. The combined effect of RSW and post-welding sensitization at 750 °C gives rise to an overall DOS that: (i) is lower than that of the state without prior welding; (ii) that decreases initially with increasing sensitization time but then increases. This behaviour is due to the fact that the interdendritic corrosion (IDC) located in the weld metal decreases with increasing sensitization time.     

Effect of biofilm on ennoblement and localized corrosion of stainless steel in fresh dam-water

AISI 304 stainless steel specimens were exposed in-situ to fresh dam-water for nearly 2 years. Open circuit potential (OCP) of the specimens becomes remarkably ennobled after exposure for about 40 days. The ennobled OCP of coupons is greatly affected by both immersion depth and sunlight. It is found that ammonium, nitrate, nitrite and chloride ions are enriched in the biofilm. It is suggested that the reduction of nitrate may be partly responsible for the OCP ennoblement. Localized corrosions at welds and crevices of specimens are attributed to both the OCP ennoblement of specimens and the chloride ion enrichment in the biofilm.     

Influence of ultrasound on pitting corrosion and crevice corrosion of SUS304 stainless steel in chloride sodium aqueous solution

The change of polarization curves and surface morphologies of SUS304 stainless steel was investigated in 3.5 mass% NaCl solution with or without the application of ultrasound (US). As the result, both the pitting corrosion and the crevice corrosion were largely suppressed by the application of US. The reason is attributed to the decrease in the concentration of hydrogen and chloride ions in pits or in the crevice by removing the corrosion product and stirring the liquid there.     

Oxidation of stainless steel 304L in carbon dioxide

Oxidation of 304L stainless steel in a carbon dioxide atmosphere at 10⁵ Pa has been studied. Between 1193 and 1293 K the oxidation kinetics exhibit first a rapid increase, then a parabolic behaviour with apparent activation energy of (209 ± 8) kJ mol⁻¹ and obeys a Langmuir pressure law. After 1.15 mg cm⁻², the kinetics become almost linear.The reaction products are chromia at the grain boundaries, wüstite (Fe_1−xO) on the surface for weight gains greater than 0.30 mg cm⁻² and chromite. The very complex reaction mechanism takes into account random buckling for weight gains >1.15 mg cm⁻².     

Influence of input power to vibrator and vibrator-to-specimen distance of ultrasound on pitting corrosion of SUS304 stainless steel in 3.5% chloride sodium aqueous solution

The pitting corrosion of SUSU304 steel can be suppressed by the application of a 19.5 kHz ultrasound (US) in 3.5% NaCl aqueous solution. At a constant vibrator-to-specimen distance of d = 76 mm equal to the wavelength, the suppression effect increased with the input power to vibrator and the largest effect was obtained at the power of I = 8. At constant input powers of I = 2 or 8, the suppression effect decreased with the increase in the vibrator-to-specimen distance, but the largest effect was obtained at d = 68 mm in each case of I = 2 and 8.     

Effect of annealing temperature on the corrosion behavior of duplex stainless steel studied by in situ techniques

The effects of the annealing temperature on the microstructure and the corrosion behavior of duplex stainless steel 2507 were investigated by means of magnetic force microscopy (MFM), scanning Kelvin probe force microscopy (SKPFM), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The SEM results indicated that the volume fraction of the austenite phase decreased with the increased annealing temperature. SKPFM/MFM measurements conducted in air at the room temperature and an ambient relative humidity of about 25% showed a higher Volta potential of the paramagnetic austenite than that of the ferromagnetic ferrite. The in situ AFM observation in a hydrochloric acid solution provided solid evidence that corrosion preferentially occurred in the ferrite phase. The sample annealed at 1100 °C exhibited a greater Volta potential difference between the ferrite and austenite and a higher corrosion rate in the ferrite, while that annealed at 1150 °C had a smaller Volta potential difference and a lower corrosion rate. The relative nobility and microstructure change of two phases, as well as their corrosion behavior, can be explained by the effect of the composition of alloying elements.     

Microstructure and pitting corrosion resistance of annealed duplex stainless steel

The transition from metastable to stable pitting was studied in 0.5 M NaCl water solution for two cast duplex stainless steels under different microstructural conditions achieved by annealing in the range from 900 °C to 1200 °C. The ensuing microstructural changes in heat treated steels were defined and correlated with established pitting potentials (E_p) and sites of corrosion damage initiation. The variations in E_p have been discussed in terms of secondary phases precipitation. The critical condition for pit stability was quantified and used to select an appropriate microstructural state, resulting in the higher potential at which stable pit growth is first observed.     

Effect of Geobacter sulfurreducens on the microbial corrosion of mild steel, ferritic and austenitic stainless steels

The influence of Geobacter sulfurreducens was tested on the anaerobic corrosion of four different steels: mild steel 1145, ferritic steel 403 and austenitic steels 304L and 316L. Within a few hours, the presence of cells induced a free potential (E_oc) ennoblement around +0.3 V on 1145 mild steel, 403 ferritic steel and 304L austenitic steels and slightly less on 316L. The kinetics of E_oc ennoblement depended on the amount of bacteria in the inoculum, but the final potential value depended essentially on the nature of the material. This effect was due to the capacity of G. sulfurreducens to create a direct cathodic reaction on steel surfaces, extracting the electrons directly from material. The presence of bacterial cells modified the corrosion features of mild steel and ferritic steel, so that corrosion attacks were gathered in determined zones of the surface. Local corrosion was significantly enhanced on ferritic steel. Potential ennoblement was not sufficient to induce corrosion on austenitic steels. In contrast G. sulfurreducens delayed the occurrence of pitting on 304L steel because of its capability to oxidize acetate at high potential values. The electrochemical behaviour of 304L steel was not affected by the concentration of soluble electron donor (acetate, 1–10 mM) or the amount of planktonic cells; it was directly linked to the biofilm coverage. After polarization pitting curves had been recorded, microscopic observations showed that pits propagated only in the surface zones where cell settlement was the densest. The study evidenced that Geobacter sulfurreducens can control the electrochemical behaviour of steels in complex ways that can lead to severe corrosion. As Geobacteraceae are ubiquitous species in sediments and soils they should now be considered as possible crucial actors in the microbial corrosion of buried equipment.     

Fatigue strength of 316L-type stainless steel in simulated body fluids

The fatigue strength of commercial SUS316L stainless steel was studied in PBS(−) (phosphate buffer solution) with different dissolved oxygen contents and CPBS (a 0.9 mass% NaCl-containing citric phosphate buffer solution) with different pH levels, i.e., 5.0, 6.0, and 7.5. The results obtained are as follows. (1) The fatigue behaviour in PBS(−) was hardly affected by the dissolved oxygen content. (2) The fatigue strength in a high-cycle region was much lower in CPBS of pH 7.5 than in PBS(−) of pH 7.5. (3) The fatigue strength in CPBS decreased with decreasing pH from 7.5 to 5.0.     

Study of the correlation between pitting corrosion and the component ratio of the dual phase in duplex stainless steel welds

Three duplex stainless steel weldments were produced by changing the chromium element to study the correlation between the pitting corrosion characteristics and the component ratio of the dual phase. The pit morphologies showed that metastable pits were generated at a lower pitting resistance equivalent number (PREN) phase. The secondary austenite phases seemed to serve as a path for the corrosive environment regardless of the ferrite number (FN). There is some discrepancy between the measured values (pitting potential (E_p) through polarization test) and expected values (sequence ranked by PREN of weaker phase) in 1 mol l⁻¹ NaCl solution at 60 °C.     

A study of intergranular corrosion of austenitic stainless steel by electrochemical potentiodynamic reactivation, electron back-scattering diffraction and cellular automaton

The impact of solution and sensitization treatments on the intergranular corrosion (IGC) of austenitic stainless steel (316) was studied by electrochemical potentiodynamic reactivation (EPR) test, and the results showed the degree of sensitization (DOS) decreased as solution treatment temperature and time went up, but it increased as sensitization temperature prolonged. Factors that affected IGC were investigated by field emission scanning electron microscope (FE-SEM) and electron back-scattering diffraction (EBSD). Furthermore, the precipitation evolution of Cr-rich carbides and the distribution of chromium concentration were simulated by cellular automaton (CA), clearly showing the effects of solution and sensitization treatments on IGC.     

Whisker growth morphology of high temperature oxides grown on 304 stainless steel

During the high temperature oxidation of as-cast Type 304 stainless steel there is a complex growth pattern of the oxide layers which is shown to be dependent on; alloy composition and local segregation, surface finish, temperature and atmosphere. Of interest is the appearance of whisker growth morphologies which grow in excess of 10 μm in length and at random orientations to the sample surface. This paper illustrates these features and discusses the formation mechanism in terms of the oxide microstructure.     

Stainless steel ennoblement in freshwater: From exposure tests to mechanisms

This work confirms the systematic character of the open circuit potential (OCP) ennoblement for stainless steels immersed in natural freshwater, and highlights the mechanism(s) responsible for this evolution. To achieve these results, electrochemical measurements and analysis/observations of the surface were realized in situ during exposure tests in the river Seine on two exposition sites using an original immersion device. Electrochemical results show that on both immersion sites a new oxidizing compound is produced close to the surface and that its reduction, occurring at potentials higher than the initial OCP value, leads systematically to the ennoblement phenomenon. Surface analysis, electrochemical and chemical tests show that the oxidizing compound is not the same on both sites: on one site hydrogen peroxide is produced within the biofilm, while on the second one oxidized manganese is deposited on the surface. Thus the two mechanisms mentioned in the literature can occur on stainless steels immersed in the same water and lead to similar evolutions of the electrochemical behaviour. These two mechanisms are not specific of a type of water and seem to be complementary rather than opposed. They are both based on enzymatic processes.     

Determination of the sensitized zone extension in welded AISI 304 stainless steel using non-destructive electrochemical techniques

Extension of sensitized zone (SZ) in welded AISI 304 stainless steel was determined by two non-destructive electrochemical tests: double loop electrochemical potentiokinetic reactivation technique (DLEPR) and local electrochemical impedance spectroscopy (LEIS). Welding was carried out using the shielded metal arc with two selected welding energies: the first one (0.7 kJ mm⁻¹) does not promote the sensitization of the 304 steel and it constitutes the reference sample and the second one (2.2 kJ mm⁻¹) which leads to the precipitation of chromium carbides in the grain boundaries after the welding process. The non-destructive DLEPR and LEIS tests allowed the length of the SZ to be determined and a good agreement between the two techniques and the microstructure of the two welded samples was shown. The presence of an inductive loop on the local impedance diagrams seems to reflect a galvanic coupling between the weld string (anode) and the welded stainless steel plates (cathode) which will be very prejudicial to a good corrosion resistance of the welded system. The results showed that the two electrochemical tests could be applied in practical cases in industrial field.     

Microstructural evolution and localized corrosion resistance of an aged superduplex stainless steel

The present investigation was undertaken to analyze the effects of isothermal ageing treatments, carried out between 700 and 900 °C for a variety of times up to two weeks and followed by water quenching, on the microstructure and on the localized corrosion resistance of a superduplex stainless steel, SAF 2507.The quantitative metallography coupled with X-ray diffraction techniques was adopted to follow the microstructural evolution, together with SEM microscopy.Electrochemical potentiodynamic tests, as cyclic polarization curves recorded in sodium chloride solutions, together with weight loss measurements were employed to evaluate the susceptibility of the aged specimens to pitting corrosion.The influence of the transformation of ferrite into secondary austenite and sigma phase and of other microstructural variations, as chromium nitrides precipitation, on the stability of the passive film is shown. The susceptibility of the aged alloy to pitting corrosion phenomena, is related to sigma phase precipitation in association to the secondary austenite formation, which lead to a noticeable Cr depletion at grain boundaries.     

Investigation of intergranular corrosion of 316L stainless steel diffusion bonded joint by electrochemical potentiokinetic reactivation

Electrochemical potentiokinetic reactivation technique (EPR) was employed to assess degree of sensitization in 316L stainless steel diffusion bonded joint (DBJ). The result showed the degree of sensitization of DBJ was much smaller than that of base material (BM). No chromium carbides precipitated at grain boundaries in DBJ after 100 h treatment at 650 °C, while chromium carbides could be seen clearly in the BM after 8 h treatment, indicating that DBJ has better intergranular corrosion resistance than BM. Diffusion bonding technique will not increase intergranular corrosion susceptibility of 316L DBJ. Reactivation potential has the biggest effect on sensitization.     

Critical pitting temperature for Type 254 SMO stainless steel in chloride solutions

The variation with time of the open circuit potential of high molybdenum containing stainless steel (Type 254 SMO) was measured in 4% sodium chloride solution in the temperatures range 30-100 °C. The plot of steady state potentials as function of temperature showed an inflection at 50 °C, attributed to the decrease of oxygen solubility in test solution above 50 °C. Potentiodynamic cycling anodic polarization technique was used to determine the critical pitting potential (E_pit) and the critical protection potential (E_prot) of the steel in 4-30% NaCl solutions at temperatures between 30 and 100 °C. By plotting the two values versus solution temperature, the corresponding critical pitting (CPT) and the critical protection (CPrT) temperatures were determined. Both parameters decreased with increasing chloride content. Above the CPT, E_pit and E_prot decreased linearly with log[Cl⁻]. The addition of bromide ions to the solution shifted both E_pit and E_prot towards positive values. In 4% NaCl, E_pit increased linearly with pH in the range 1-10. The combined effect of chloride ion concentration and pH on the morphology of the pits was examined by scanning electron microscopy (SEM) following potentiodynamic cycling anodic polarization.