I. Chattoraj

Corrosive degradation and failure of vertical furnace wall tubes of a boiler

The failures of vertical furnace wall tubes of a co-generation boiler have been investigated. The thinned section failures, often accompanied by buckling prior to failure, were found to be due to acid attack as a consequence of “hideout”. The local corrosion and consequent thick and non-protective iron-oxide deposition at the sites of attack produced hot spots, which is evident from the microstructure of those regions. Decarburization by hydrogen generated during the corrosion process also contributed to the loss in strength of the material. The possibility of “alkali attack” as opposed to acid attack was ruled out by the various findings of this investigation.

Investigation on the mechanical degradation of a steel line pipe due to hydrogen ingress during exposure to a simulated sour environment

The loss of ductility of a longitudinally welded steel pipe, brought about by hydrogen ingress during exposure to simulated sour environments, was investigated. The material was found to be very prone to hydrogen induced cracking (HIC). The presence of hydrogen-induced cracks were not necessarily detrimental to the ductility as demonstrated by ex-situ straining. It was concluded that mobile lattice hydrogen was responsible for ductile loss, while trapped hydrogen was responsible for HIC.

Anodic reactions of amorphous and devitrified Fe–B–Si–Nb–Cu alloys in buffered chloride and fluoride

The anodic response of two Fe–Si–B–Cu–Nb alloys was investigated in buffered chloride and fluoride media through polarisation studies and corrosion product analysis. It was found that the anodic reactions and dissolution of the alloys were dependent on the alloy compositions as well as the state (amorphous or crystalline) of the alloy substrate. It was observed that crystallisation resulting in the formation of the ordered Fe80Si20 phase only, was beneficial to corrosion resistance in chloride media, however the presence of secondary intermetallic phases was detrimental. The anodic responses in the fluoride media were dominated by the formation of fluorosilicate.

Effects of Cold Deformation Prior to Sensitization on Intergranular Stress Corrosion Cracking of Stainless Steel

Abstract The effects of deformation, prior to sensitization, on intergranular stress corrosion cracking (IGSCC) were studied on the AISI 304 (UNS S30400) stainless steel (SS). The degree of sensiti...

Hydrogen induced brittle crack growth in Cu-strengthened HSLA-100 steels

The annular brittle cracking in notched samples of an HSLA-100 steel has been studied by crack interrupts. The average crack growth rate was found to be geometry and strain rate dependent. Suitable normalisation led to a generalised expression for crack growth rates that was strain rate and geometry independent. The crack growth rates were found to be directly proportional to a ‘critical product’ namely {KIcrack-tip·CHcrack-tip}. The crack initiation threshold for notched specimens showed an inverse exponential relationship with the notch tip hydrogen concentration. The crack tip hydrogen concentration was best represented by a hydrostatic stress equilibrated concentration for shallow notched specimens. Hydrogen deficiency in deep notches necessitated the use of a ‘frozen-in’ concentration approach for determining the crack tip hydrogen.

The association of potentiokinetic reactivation and electrochemical pitting tests on a nitrogen bearing 19 Cr-17 Mn steel with its thermal history

Abstract The double loop electropotentiokinetic (EPR) technique was used to study the transformations in a 19 Cr-17 Mn steel. This technique revealed that the transformation of delta ferrite and other microstructural variations during cooling after solution annealing influenced the stability of the passive film formed on a nickel substituted stainless steel exposed to aggressive media. The pitting susceptibility of the Cr Mn steel was found to have increased significantly after being subjected to furnace cooling following solution annealing. The presence of delta ferrite in the austenite matrix was found to have a benign influence on pitting resistance. The formation of a sigma phase was found to be responsible for the deterioration of corrosion resistance. This paper explains the variation in the EPR test results by correlating the changes in the alloy microstructure brought about by thermal history with the changes in electrochemical responses, the latter being directly influenced by the matrix and grain boundary Cr content of the different phases formed.

Effect of plasma ion implantation on the hydrogen embrittlement of Cu strengthened HSLA-100 steel

The effect of low dosage plasma ion implantation on hydrogen embrittlement was studied for an HSLA steel using notched tensile samples. The plasma treatment caused an enhancement in the linear strain to failure under embrittling conditions. This was however not reflected in the fracture surfaces of the treated samples which had similar fractographic features as those of untreated samples. The plasma treatment delayed the process of embrittlement without causing any alteration in the basic mechanism of embrittlement. This was due to introduction of residual compressive stresses as well as reduction in the hydrogen permeation flux. Implantation in pure nitrogen seemed most beneficial while implantation in pure argon caused very little improvement.

Self-similar and self-affine characteristics of microstructural images of HSLA steel

Abstract This paper reports a procedure based on discrete Fourier transformation adopted for verifying statistical self-affinity and self-similarity in microstructural images of high strength low alloy steel. The procedure is first applied on synthetic images based on fractional Brownian motion to show the degree of self-similarity in them and finally, applied on microstructural images of three different magnifications. Effects of length scales on power spectral estimates of the microstructural images captured by scanning electron microscopy have been discussed. Importance of appropriate image resolutions and characteristic length for fractal based quantification of microstructural images is highlighted.

Stress corrosion cracking in an Al-bronze chlorine gas regulating valve

An analysis of the failure of a dry chlorine gas regulating valve made of Al-bronze is presented. The valve had a stainless steel spindle within its chamber. Cracks had initiated in the inner chamber walls and proceeded outwards resulting in leakage. The corrosion products on the crack surface were found to be a chloride–hydroxide complex of copper. This suggested the ingress of moisture along with the gas. The corrosion mechanism was identified as complex galvanic coupling between the dissimilar alloys which also resulted in depassivation of the spindle surface. The stresses for crack propagation had come from the contact stresses as well as gas pressure on engagement of the spindle with the valve body.

Passivity breakdown due to discontinuous precipitation during ageing of 21Cr-10Mn-5Ni stainless steel

Discontinuous precipitation in a 21Cr-10Mn-5Ni steel was found to result in breakdown of passivity when tested in a 0.5 m H2SO4 + 0.01 m KSCN solution. The electrochemical response was similar to that due to sensitization of conventional stainless steels. The ageing time and temperature affected the resistance of the passive film, with lower ageing temperatures being more severe in terms of passivity breakdown. The effect of interlamellar spacing on the chromium depletion, the effect of volume diffusion on the healing of the matrix, and the solubility product reaction for precipitation, were considered to explain the observed different strengths of passive films formed on specimens aged at different temperatures and times. The observance of good corrosion protection in spite of the presence of profuse precipitates was in contrast to the effect of grain-boundary carbide proliferation on the corrosion resistance of conventional stainless steels.