Effect of aqueous LiBr solutions on the corrosion resistance and galvanic corrosion of an austenitic stainless steel in its welded and non-welded condition

The corrosion resistance and galvanic behaviour of a highly alloyed austenitic stainless steel (UNS N08031) and a nickel-base alloy (UNS N06059) in its welded (GTAW) and non-welded condition were analysed in LiBr solutions by means of electrochemical measurements. Samples microstructure was studied by SEM and EDX analysis. The alloys considered showed passive behaviour and they were able to repassivate after the breakdown of the passive film. The pitting resistance of the materials increased as the LiBr concentration decreased. With regard to galvanic corrosion, the welded metal was always the anodic element of the UNS N08031-welded metal pair.     

Fatigue performance of a SAE 1045 steel coated with a Colmonoy 88 alloy deposited by HVOF thermal spraying

The present investigation has been conducted to study the fatigue behavior of a SAE 1045 steel both uncoated and coated with a Colmonoy 88 alloy (NiCrBSiW) of approximately 410 μm thick, deposited by HVOF thermal spraying. Previously to deposition the samples were grit-blasted with alumina particles of approximately 1 mm in equivalent diameter. Tensile and fatigue tests were carried out with the uncoated and coated specimens. Fatigue tests were conducted under rotating bending conditions (R = − 1) at a frequency of 50 Hz. The samples tested were in three different surface conditions, including polished, grit-blasted and coated. The fatigue limit was determined by means of the staircase method employing a stress step of 5 MPa. The results indicate that the presence of the coating gives rise to a reduction in the fatigue life of the coated samples tested in air in comparison with the uncoated specimens. On the contrary, when the coated samples were tested in a NaCl solution at alternating stresses less than 350 MPa, these showed an increase in fatigue life in comparison with the polished uncoated samples. The analysis of the fracture surfaces of the specimens tested in air revealed that alumina particles present on the surface of the grit-blasted samples acted as stress concentrators, inducing the nucleation of fatigue cracks at the substrate-coating interface, which explains the reduction in fatigue life. However, under corrosive conditions and low alternating stresses, the presence of the coating provides an effective protection against corrosion-fatigue failures, giving rise to an improvement of the corrosion-fatigue performance of the coated system. On the contrary, at elevated alternating stresses, the coating was observed to delaminate from the substrate, leading to an impairment of the corrosion-fatigue behavior of the coated samples.     

Reheat cracking studies on simulated heat-affected zones of CrMoV turbine rotor steels

The reheat cracking susceptibility of the heat-affected zone (HAZ) of a CrMoV turbine rotor steel was investigated. Two base materials, one with a coarse-grain (155 μxm) and the other with a fine-grain (55 μim@#@) microstructure, were submitted to Gleeble HAZ weld simulations. Three peak temperatures were utilized: 1350,1150, and 950 °. Some samples were single cycled, and others were exposed to a double cycle. The samples that were double cycled experienced a second peak temperature 150 to 250 ° lower than the first peak temperature. The samples were then stressed in bending for different amounts and stress relieved under load to determine their reheat cracking susceptibility. All samples were metallurgically evaluated before and after the reheat cracking test. It was found that the prior-austenite grain size of the original base metal did not influence the reheat cracking susceptibility, but increases in peak temperature did. It was observed that the grain size and grain matrix microhardness that developed after the Gleeble cycles affected reheat cracking. It was found that reheat cracking did not occur when the microhardness was below 350 DPH and the prior-austenite grain size was less than about 80 μm.     

Corrosion behavior of engineering alloys in synthetic wastewater

The corrosion behavior of 1018, 410, and 800 steels exposed to synthetic wastewater have been studied using linear polarization resistance, cyclic potentiodynamic curves (CPCs), electrochemical noise (EN), and electrochemical impedance spectroscopy (EIS) tests. The conditions were: a biochemical oxygen demand of 776 ppm; a chemical oxygen demand of 1293 ppm; a pH of 8; and a cell temperature of 25 °C. From the CPC and EN results, no localized corrosion was found for the stainless steels. However, small indications of a possible localized corrosion process were detected for the 1018 steel. The EIS results revealed that different corrosion mechanisms occurred in the carbon steel compared with the stainless steels. The results show that the corrosion mechanism strongly depends on the type of steel. Overall, the 1018 steel exhibited the highest corrosion rate, followed by the 410 alloy. The highest corrosion resistance was achieved by the 800 alloy. In addition, scanning electron microscopy analyses were carried out to explain the experimental findings.     

Carburization kinetics of heat-resistant steels

A series of 21 commercial austenitic heat-resistant alloys was carburized in H₂-C₃H₆ atmospheres which were reducing to Cr₂O₃ and SiO₂ and which provided a carbon activity of one. Reactions were carried out at 900T1100°C and the resulting depth of carburization was measured metallographically. All alloys showed parabolic carburization kinetics after an initial brief period, at low temperatures, of more rapid reaction. The apparent activation energies for carburization are discussed in terms of carbon solubilities and diffusion coefficients. It is concluded that Wagner's theory of internal oxidation is quantitatively applicable to simple alloys but not to alloys which contain additions of reactive elements. The effectiveness of additions of niobium, aluminum, titanium, and silicon is demonstrated and is discussed in the context of Wagner's theory.     

Corrosion of carbon steels in high-temperature water studied by electrochemical techniques

Electrochemical methods were used to study the corrosion behavior of five Fe-Cr alloy steels and 304L stainless steel in high-temperature water. Passivity can be achieved on A-106 B carbon steel with a small content of chromium, which cannot be passivated at room temperature. The formation rate and the stability of the magnetite film increased with increasing Cr content in the steels. A mechanistic model was developed to describe the corrosion processes of steels in high-temperature water. The crack growth rate on steels was calculated from the maximum current of the repassivation curves according to the slip-oxidation model. There was a high crack growth rate on 304L stainless steel in high-temperature water. Of the four Fe-Cr alloys, the crack growth rate was lower on 0.236% Cr- and 0.33% Cr-steels than on 0.406% Cr steel and 2.5% Cr-1% Mo steel. The crack growth rate on 0.33% Cr steel was the smallest over the tested potential range.An increase in Cr content in the steel is predicted to reduce the corrosion rate of steel at high temperatures. However, this increase in Cr content is predicted not to reduce the susceptibility of steel to cracking at high temperatures.     

The corrosion wear behaviour of selected stainless steels in potash brine

Tests were conducted at 25 and 85 °C to evaluate the corrosion wear resistance of selected stainless steels in potash brine using a reciprocating motion wear apparatus. Four materials were tested: Ferralium 255 (UNS S32550), AL6XN (UNS N08367), 254SMO (UNS S31254) and AISI 1018 (UNS G10180) for comparative purposes. The evaluation methods employed included weight loss analysis, optical microscopy and scanning electron microscopy (SEM). The results show that Ferralium 255 has superior corrosion wear resistance in potash brine environment compared to AISI 1018 plain-carbon steel and the other stainless steels tested. Wear surface analysis using SEM shows evidence of brittle fracture damage, which is attributed to the presence of Cl⁻.     

Corrosion behavior of AISI302 steel treated by elevated temperature nitrogen plasma immersion ion implantation

The corrosion behavior of AISI302 steel implanted with nitrogen at elevated temperature was investigated by electrochemical impedance spectroscopy. Equivalent circuits for explaining the impedance characteristics are proposed. The thick passive layer containing Cr₂O₃ and the expanded austenite layer in the sub-surface worked together, resulting in the high corrosion resistance.