Pitting and sulphide inclusions in steel

The initiation of pitting corrosion at MnS inclusions

Electrochemical measurements were performed on single sulfide inclusions within a microcell to study their dissolution behavior and role in the initiation of pitting corrosion of 304 stainless steel in NaCI solutions. It was found that chloride ions catalyzed inclusion dissolution and caused metastable pits and that no metastable pits formed in thiosulfate solutions without chloride. The thiosulfate ion accelerated inclusion dissolution. and when the thiosulfate ion was present above a critical concentration it caused stable pitting, at low concentrations the chloride ion inhibited the aggressive nature of the thiosulfate ion and stifled stable pitting. while at high concentrations the thiosulfate ion inhibited the ability of the chloride ion to cause metastable pitting events. The addition of xanthate ion in combination with electrode activation in copper sulfate was found to inhibit the rate of inclusion dissolution and increase the pitting potential. The initiation of stable pitting was found to depend on the inclusion geometry.

Pit Initiation at Single Sulfide Inclusions in Stainless Steel I. Electrochemical Microcell Measurements

The birth of corrosion pits as stimulated by slurry erosion

The origins of persistent interactions among localized corrosion sites were investigated using multielectrode arrays (MEA). MEAs consisted of one hundred 250 μm diam AISI 316 stainless steel wires configured into 5 X 20 close-packed rectangle and 5 X 5 far-spaced configurations. Experiments were conducted in dilute chloride solutions and elevated temperatures to accentuate interactions. Interactions between early dominating pits and the adjacent electrode surface were found to develop as regions of enhanced or suppressed pitting susceptibility triggered by concentration and potential fields developed during growth of large pits, respectively. Transient aggressive species accumulation around active pits was also found to cause surface damage to electrodes that persisted for long periods after primary pits were repassivated and solutions extensively stirred. Oxide film alteration, mixed metal sulfide inclusion damage, and surface contamination were all considered to be possible origins of persistent interactions. Mn(Fe,Cr)S inclusions were implicated to be the origin of such persistent interactions. Persistent interactions were completely suppressed by nitric acid treatment, which is believed to remove or modify susceptible Mn(Fe,Cr)S inclusions.

Origins of Persistent Interaction among Localized Corrosion Sites on Stainless Steel

The history of dynamic straining in stress corrosion cracking studies and the evolution of the slow strain rate test (SSRT) are reviewed. Smooth and notched specimens; the importance of strain rate, electrode potential, and other environmental factors; the evaluation of test results; and comparisons to other techniques are addressed. The SSRT’s application in research for oil and gas sour service, buried natural gas pipelines, ethanol transportation, nuclear power, low pressure turbines, and mechanism studies is summarized and its usage by material, industry, and geographic region quantified. Standard test procedures are compared and improvements suggested. The more recent use of cyclic loading is discussed and areas for future study proposed.

The Slow Strain Rate Stress Corrosion Cracking Test—A 50 Year Retrospective

The influence of manganese (0.3 to 2.4%), sulfur (0.01 to 0.4%), sulfide composition, and heat treatment on the corrosion resistance of 13% chromium, 17% chromium and 18% chromium-9% nicke...

Corrosion of Resulfurized Free-Machining Stainless Steels

Weldments sometimes corrode preferentially in the weld metal itself or in the heat affected areas of the parent metal. The localized nature of the attack limits the usefulness of standard weight loss corrosion tests. Techniques for overcoming this limitation are discussed. The influence of specimen geometry and surface finish are dealt with as well as a variety of destructive and nondestructive methods for evaluating a corroded weldment. The advantages and limitations of the various ASTM intergranular tests for austenitic stainless, ferritic stainless, and nickel base alloys are reviewed. Guidelines for evaluating weldment corrosion are given, although it is recognized that there is no single best procedure for all situations. The use of sensitization treatments to simulate welding is discussed.

Michael Henthorne

Papers

The Slow Strain Rate Stress Corrosion Cracking Test—A 50 Year Retrospective

Corrosion of Resulfurized Free-Machining Stainless Steels

Corrosion Testing of Weldments