Chloride Stress Cracking Mean?
Chloride stress cracking is very common in chemical processing and petrochemical industries. It is considered the prime reason for the deterioration of austenitic stainless steel pipework and vessels used in the petrochemical industry. Damage caused by CI-SCC is easily recognizable from the highly branched cracks that look like spider webs or lightning-array type network visible on the material.
Cl-SCC has caused many pipeline failures and it continues to be a problem. This is primarily because of unanticipated contamination of equipment made with austenitic steel due to inadvertent chloride contamination. Design engineers should be aware of the potential consequences of using austenitic stainless steel where chlorides may be present. The high cost of replacing equipment can be economically devastating for some industries.
Austenitic stainless steel is an iron-based alloy that contains 19% chromium and 9% nickel. This steel is highly resistant to corrosion in most atmospheric and aqueous environments. However, wet and humid environments containing chloride ions can cause pitting and crevice corrosion of austenitic steel components. Furthermore, components that are under applied or residual stress can, under these conditions deteriorate further by stress corrosion cracking.
To reduce the risk of Cl-SCC, the plant and equipment design should be such that the concentration of mechanical tensile stress such as at sharp edges and notches is avoided. Proper material selection is also important though it is true that none of the stainless steel grades is completely resistant to chloride stress corrosion cracking.
Chloride stress cracking is also known as chloride stress corrosion cracking (CI-SCC).
Trenchlesspedia Explains Chloride Stress Cracking
A chloride contaminated corrosive environment combined with tensile stress can cause stainless steel to crack. To prevent equipment from succumbing to Cl-SCC, the components should be designed by lowering the stress and using resistant alloys.
The resistance of austenitic steel to Cl-SCC attack depends on the nickel content present in it. Austenitic steel grades 304/304L and 316/316L most susceptible to Cl-SCC as they have lower levels of nickel content. Austenitic grades with higher nickel content such as alloy 20, 904L and the 6% molybdenum super austenitic grades are better resistant to Cl-SCC.
Following are some of the places where the chloride cracking of 300 series stainless steel can take place:
Non-destructive examination (NDE) to detect Cl-SCC
Dye-Penetration Testing (PT)
The surface under inspection is required to be clean, accessible, and visible to the inspection personnel. Works best when both surfaces can be inspected and finding through-wall Cl-SCC can be increased by applying dye-penetrant on one surface and developer on the opposite surface.
Eddy Current Testing (ECT)
This kind of testing is used for heat-exchanger tubing and pipework where there is long-distance consistency of surface condition and geometry. However, it can only find surface-breaking cracks or cracks just below the surface.
Ultrasonic Flaw Detection (UT)
Since weld grains in fabricated and welded austenitic steel are coarse-grained, they give false signals and high sound attenuation, requiring a specialized technique. It can be applied to a surface where cracks have initiated or from the opposing surface.