Crevice corrosion poses a threat to offshore environments such as oil rigs and desalination plants in part because it is difficult to detect. Unlike pitting, which can often be seen with the naked eye, crevice corrosion occurs in the hidden space between metal couplings. Because it is difficult if not impossible to observe, it poses one of the greatest threats to offshore structures.
When two otherwise non-corrosive metals are joined in a single environment, a gap exists at the point where they are connected. This space creates conditions ideal for corrosion to occur. As seawater gathers and stagnates in a crevice, the profuse number of chloride ions not only generate corrosion but also interact with the water and metal ions to produce acid, redoubling the attack. The tighter the space, the greater the vulnerability as wider spaces allow for circulation of fluids, preventing stagnation.
Another problem common in offshore environments is contamination. Welding generates iron particles. Drilling and handling lead to surface deposits. Diesel exhaust contains high levels of sulfur. Each of these contribute to the localized deterioration of joined metals.
Tubing installations, which are used in a wide array of process instrumentation, present a particular risk as crevices are nearly unavoidable when connecting tubing and tube fasteners. Once crevice corrosion is instigated, it’s a matter of time before it leads to a crevice failure and stress cracking.
Because this is such a prevalent problem in the offshore industry, innovators from around the world have focused on means to mitigate it. Choosing certain nickel and ferritic alloys that stand up to seawater better is a logical first step. For less resistant alloys, thermoplastic polyurethane jackets can be applied to tubes to increase their integrity.
Likewise, the development of tubing brackets that resist corrosion can significantly reduce the opportunity for crevice corrosion to occur. At PFSNO, we’ve modified a traditional T-SEP bracket, making it a superior choice for offshore fabrication, and in our next post, we’ll show you how we did it.