Learning from Failures

The boiler is the heart of a refinery or power plant. Boiler failures cause forced shutdowns and put the health of the plant at risk. 

Some boiler designs run the risk of thermal fatigue failure. During service, the internal waterside surfaces of the tubes form a thin, protective oxide layer. Yet if the strains caused by the combination of (1) thermal expansion and (2) restraint during temperature fluctuations exceed the oxide strain capacity, the oxide cracks. Each thermal cycle will repeatedly crack the oxide, expose fresh metal when then oxidizes, eventually forming a crack. Technically this mode of cracking is corrosion fatigue-driven by thermal cycling. Yet since the boiler water will always be mildly corrosive, for boiler applications this mode of cracking is simply called “thermal fatigue”.

In the previous Learning From Failure article, we reviewed that chloride stress corrosion cracking (Cl-SCC) of stainless steel readily occurs at temperatures above 140°F (>60°C) when exposed to aqueous (water-based) chlorides.  Although the most attention is given to corrosion from product exposure, Cl-SCC can also occur from the external surfaces as a result of corrosion under insulation (CUI).  Here is an example. [Read more…]

It is well known that chloride stress corrosion cracking (Cl-SCC) of stainless steel is caused by the combination of (1) aqueous chlorides, (2) stress and (3) temperature. Yet even with great awareness, Cl-SCC still manages to sneak up and cause many surprise failures. Here is a failure analysis case study to summarize the key factors causing this corrosion cracking mode. [Read more…]