Critical Stress Corrosion Cracking Potentials of Stainless Steels in Dilute Chloride Solutions

Stainless steels are susceptible to stress corrosion cracking (SCC) in wide range of environments. SCC occurs predominantly when stainless steels are exposed to chloride containing environments. The concentration of these chloride ions vary with different industrial applications from 10ppm in cooling water to 22,000ppm (seawater). This study was focussed on low chloride concentrations and is relevant to steam turbines and cooling water systems.

            The effect of applied potential (E_app) on (SCC) behaviour of stainless steels (12%Cr - 25%Cr, PREN 11- 43) was investigated using the slow strain rate technique (SSRT) in an autoclave with controlled potentials, using a three electrode cell and potentiostat. The tests were conducted at 130⁰C in a solution containing 15-30ppm chloride and 8ppm dissolved oxygen. The results showed that the reduction in ultimate tensile strength (UTS) and the reduction in area (RA) varied with the applied potential. A critical SCC potential (E_SCC) was found to exist in a narrow potential range for each of the materials tested

.             Fractographic observations on fractured specimens in the scanning electron microscope (SEM) showed that the loss of ductility, indicated by the decrease in RA was linked to the initiation, coalescence and propagation of cracks for all materials. Depending on the microstructure the forms and causes of SCC in all these materials was different. Safe operating limits in dilute chloride conditions were fully determined for all grades investigated in this study. The E_SCC value for given steel is related the pitting resistance equivalent number (PREN) and varied by more than 0.85V across the range of steels studied. Increasing Cr, Mo, N content increases SCC resistance, but all steels crack above E_SCC. Most SCC problems for higher PREN steels at low chloride levels can be controlled by ensuring E_SERVICE < E_P (Pitting potential). The use of high PREN steels could eliminate chloride SCC. This paper also emphasis the observed corrosion behavior by evaluating local crystallographic information, grain size, and plastic strain determined using Electron Back Scattered Diffraction (EBSD) across range of materials studied.