Unni Steinsmo

Factors affecting initiation of pitting corrosion in super martensitic stainless steel weldments

Abstract In the present investigation optical microscopy, in combination with sputtered neutral mass spectrometry (SNMS), has been used to examine the conditions for initiation of pitting corrosion in the heat affected zone (HAZ) of two super martensitic (SM) stainless steel weldments. It is shown that the corrosion resistance depends mainly on the nature of the surface oxide, as determined by the oxygen potential in the shielding gas and the HAZ temperature–time relationship, and less strongly on the underlying microstructure. In the absence of H2S the initiation is associated with spalling of the iron enriched chromium oxide within a narrow region approximately 6 mm from the fusion line, where the contaminated oxide layer is thinnest. As H2S is introduced, the region close to the fusion boundary becomes susceptible to localised corrosion because of the more extensive metal oxidation. Thus, the high temperature oxidation and iron oxide scale formation accompanying the welding operation appear to be the main factors affecting pitting corrosion initiation in SM stainless steel weldments.

Aspects of testing and selecting stainless steels for seawater applications

Abstract Relatively recent failures in seawater systems of highly alloyed (i.e., pitting resistance equivalent [PREN] > 40) stainless steels (SS) have highlighted the issue of corrosion testing and safe-use limits. Three aspects of testing and selection of SS for seawater application were reviewed: relevancy of the electrochemical test methods, the quality control system, and the importance of repassivation. A potentiostatic test for assessment of critical pitting (CPT) and crevice (CCT) temperatures was analyzed with respect to its ability to generate data relevant to chlorinated seawater systems. CPT seemed valid as safe-use limits, but the CCT determined were too high. A modified test approach was proposed. Generally, significant scatter in CCT and CPT values was registered, focusing on the importance of statistical experimental approach and data analyses. Recommendations were made on use of the ASTM G 48 test as a quality control method. The validity of critical pitting and crevice indices was analyze...

Cathodic disbonding of paint films : transport of charge

Abstract Cathodic disbonding and charge transport through paint films were studied as a function of polarization potential, temperature, dry-film thickness and cation type in the electrolyte. It was found that both the disbonding rate and the charge transport were linearly related to the polarization potential. The activation energy for the disbonding process and the charge transport were quite different. The disbonding rate was linearly related to the dry-film thickness and was dependent on the cation type in the electrolyte. This was not the case for the charge transport. The results indicate a rate controlling factor for the disbonding process involving transport of cations through the film where the film is already disbonded. A fundamental difference in the conductive properties of the paint film on either side of the disbonding front is also proposed.

Factors Influencing the Rate of Cathodic Disbonding of Coatings

Abstract Cathodic disbonding on coated steel panels was studied. The coating was applied by airless spraying on blast-cleaned steel surfaces. The importance of electrolyte, polarization potential, ...

Dissolution and Repassivation Kinetics of a 12.3Cr-2.6Mo-6.5Ni Super Martensitic Stainless Steel A Comparative Study

In this investigation, the dissolution and repassivation kinetics of a super martensitic (SM) stainless steel (Fe-12.3Cr-6.5Ni-2.6Mo) have been characterized using the artificial pit technique. As a part of this study, a diffusion model has been developed and employed for calculation of the pit surface concentration of dissolved species during the potential step experiments. For concentrations close to the saturation level the dissolution kinetics are adequately described by a Tafel slope of approximately 57 mV/dec and a current density of 0.5 mA/cm 2 at -300 mV vs. saturated calomel electrode. However, repassivation of the active pit surface occurs when the concentration of the dissolved species drops below 30% of the saturation value. Based on a comparison with relevant literature data, the observed response of the SM stainless steel to localized corrosion is similar to that reported for other high alloyed steels. This result is expected if the dissolution and repassivation kinetics are controlled by the content of Cr, Ni, and Mo in the parent material.

Critical Crevice Temperature for High-Alloyed Stainless Steels in Chlorinated Seawater Applications

Eleven high-alloyed stainless steels (SS) were tested for application in chlorinated seawater. Critical crevice temperatures (CCT) were determined using a potentiostatic test method. Results were evaluated in terms of the critical crevice index (CCI) value of the alloys and compared to results of duplicate specimens in other tests.