Stress Corrosion Cracking (SCC)
What is Stress Corrosion Cracking ?
Stress corrosion cracking (SCC) is the growth of cracks in a corrosive environment. It can lead to unexpected sudden failure of normally ductile metals subjected to a tensile stress, especially at elevated temperatures. SCC is highly chemically specific in that certain alloys are likely to undergo cracking only when exposed to a small number of chemical environments. The chemical environment that causes stress corrosion cracking for a given alloy is often one which is otherwise only mildly corrosive to that metal. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for stress corrosion cracking to go undetected prior to failure. Stress corrosion cracking often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure.
Where does stress corrosion cracking occur?
Certain austenitic stainless steels and aluminum alloys crack in the presence of chlorides. Mild steel cracks in the presence of nitrates and alkali, as is the case with boiler cracking.Copper alloys crack in ammoniacal (containing amine) solutions (season cracking). As a result, SCC limits the usefulness of commonly used austenitic stainless steel for containing water with higher than a few ppm content of chlorides at temperatures above 50 °C.
How is stress corrosion cracking avoided?
Stress corrosion cracking is the result of a combination of three factors – a susceptible material, exposure to a corrosive environment, and tensile stresses above a threshold. If one eliminates any one of these factors, SCC initiation becomes impossible. When the chloride content of the media or the stress in the material cannot be eliminated, the conventional approach to controlling the problem is to use SCC-resistant alloys. This approach is a costly proposition and can require a massive time investment to achieve only marginal success.(1)
An alternative approach is to protect existing metal surfaces from the corrosive environment with a thin layer of a material that is not susceptible to stress corrosion cracking. Thermal spray coating materials such as UTEx 5-420 and UTEx 5-620, both NiCrMo superalloys, are an effective, permanent means of mitigating stress corrosion cracking. Proprietary materials in these alloys improve pitting resistance, reduce stress, and eliminate permeability. This cladding is only effective if applied with High Velocity Arc Spray (HVAS) systems. Other wire-based systems, such as traditional Twin Wire Arc Spray (TWAS) and previous generation High Velocity Continuous Combustion (HVCC) systems produce permeable coatings, with a high degree of interlinked porosity, particularly suscept in the case of chlorides.
(1) ASM International, Metals Handbook (Desk Edition) Chapter 32 (Failure Analysis), American Society for Metals, (1997) pp. 32-26.