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 focused on low chloride concentrations and is relevant to steam turbines and cooling water systems.
The effect of applied potential (Eapp) 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 of area (ROA) varied with the applied potential. A critical SCC potential (ESCC) 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 ROA 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 ESCC value for given steel was related to the pitting resistance equivalent number (PREN) and varied by more than 0.80V across the range of steels studied. Increasing Cr, Mo, N content increases SCC resistance, but all steels crack above ESCC. Most SCC problems for higher PREN steels at low chloride levels can be controlled by ensuring ESERVICE<EP (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, and plastic strain determined using Electron Back Scattered Diffraction (EBSD) across range of materials studied.
Stainless are selected for various applications based on their strength and corrosion resistance. The impact of corrosion makes them distinguish, and pitting resistance equivalent number (PREN) plays an important role for material selection. Since these materials are used in very significant applications like power generation, Oil & Gas, Paper and pulp, it is very important to rank the materials and assist material selection. It has been reported that almost 30% of cases of the corrosion failures in power stations is caused by localised corrosion including pitting and stress corrosion cracking (SCC) 1.
SCC of stainless steels continues to be one of the important failure mechanisms in corrosion related problems. Since SCC mechanism involves initiation and propagation of cracks, it is important to detect or identify critical conditions that can promote this process. There is extensive knowledge of SCC in stainless steels, but much of the data is in sea water or boiling MgCl2, CaCl2.
