Mistras - Stress Corrosion Cracking

Stainless steel forms a passivation layer, protecting itself from corrosion. There are a number of working environments to which stainless steels are exposed which can breach this protective barrier (and a number of metallurgical theories); combined with stress levels experienced in normal service, stress corrosion cracking can begin and go unnoticed until visible cracks appear, by which time the steel is sensitised and irreparable. When SCC is detected visually in this way, production can be interrupted for many weeks while replacement plant is procured and introduced.

AE-the key to early detection and monitoring.

Acoustic Emission can detect the early stages of stress corrosion cracking. Once present, SCC is difficult to eradicate, and is otherwise detectable only by sectioning and microanalysis. AE inspection can identify and locate SCC before leaks occur, providing plant management the opportunity to plan replacement components prior to major failures and process interruptions. Active SCC and corrosion pitting is detectable on-line using AE, and the effectiveness of any steps taken to stop the process may be found by re-testing or continuous monitoring.

Case Studies

1. The photograph shows one of several cracks identified in a stainless steel column. By the time SCC becomes this evident, much of the surrounding material is sensitised with thousands of micro-cracks (as illustrated above). Attempts to repair SCC-damaged stainless by welding fail, since the heat from the weld opens these micro-cracks into macro-cracks.

In this example, the plant suffered weeks of downtime while attempts to repair the vessel were made in vain. Down-time continued until replacement vessels could be installed.

2.

560 metres of new 8-inch diameter stainless pipe-work under rockwool insulation was mistakenly hydro-tested using tap water. When the line came to be filled with acid, contents were observed leaking. Tap water remaining in the line after the hydro-test left areas of concentrated chlorides where water had lingered after drainage. An AE test to evaluate the entire line was performed. The AE analysis was able to determine which areas were affected by SCC and which were not, without which all of the pipeline would have been suspect.