Jacketing or a weather barrier is usually installed in insulated piping systems and pressure vessels to prevent water ingress and protect the insulation. In the event of water penetration and accumulation resulting from poor design and/or aging of the jacketing, drain holes are proposed to accelerate the water dry-out process. This study investigated the influence of jacketing and drain holes on corrosion under insulation of carbon steel. In an enclosed system without the drain holes, where the water dry-out process is reduced, the underlying steel remains exposed to the corrosive environment for a longer period of time. Severe pitting corrosion was observed, which could be a result of limited oxygen diffusion into the insulation creating differential aeration environments favorable for pitting corrosion. The presence of drain holes promoted the water dry-out rate and oxygen diffusion into the insulation. As a result, the average corrosion rate increased in short-term exposure tests, but over a longer term the average corrosion rate, as well as the pitting rate, decreased.

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