No abstract

The initiation and propagation of stress corrosion cracking (SCC) of austenitic stainless steels in boiling 35% magnesium chloride solution, with the pH adjusted to 3.0, were investigated electrochemically by separating the crack anode from the cathode. The dissolution current accompanying crack propagation can be measured, by connecting the anode with the cathode through an external electric circuit, and as a result, the distinction between crack initiation and propagation becomes very clear. The passive-active cell of SCC, simulated by keeping the crack anode potential at the potential of the cathode using a potentiostat, prevents the coupled potential from falling during crack propagation step, accelerates the dissolution of crack anode, and reduces the time to failure. For materials and environments which have low susceptibility to SCC, it can clearly be determined whether SCC occurs or not, by comparing the critical potential for crack propagation, not for initiation, with the corrosion potential of the unstressed steel in the environment concerned. If the former is more active than the latter, SCC can occur. The present method was applied to predict SCC in some chemical equipment, and successful results were obtained.

No abstract

A newly developed hardfacing Co-bace, carbon-free alloy, Tribaloy, was considered to apply to a continuous decanter centrifuge made of SUS 316L stainless steel as a material of the screw conveyor flight edge. Stellite, a Co-base alloy containing large amount of carbon, has been used conventionally, and suffered from intergranular corrosion on the stainless steel adjacent to the deposited hardfacing layer. Wear and corrosion resistant characteristics, and the change of the stainless steel overlayed with the hardfacing alloys, were investigated by measuring hardness, anodic polarization curve, optical microstructure, XMA, and EPMA. The following conclusions were obtained: (1) Wear and corrosion resistance of Tribaloy is superior to Stellite in a corrosive environment containing hydrochloric acid and chloride. (2) In contrast with Stellite, Tribaloy does not cause sensitization to a base metal of low carbon type stainless steel even though overlayed by welding with oxyacetylene gas flame as well as TIG welding. (3) Tribaloy has an effect to prevent penetration of carbon from the gas flame to the stainless steel. (4) This material has been applied to a practical centrifuge handling a corrosive slurry, and succeeded to prevent erosion and intergranular corrosion over three years, instead that Stellite had a life of one year or less.