Tubes of grade UNS S31603, N08904, N08825 and S32750 have been exposed to atmospheric corrosion conditions in coastal, marine environments. Two test sites have been used, one in temperate climate on the Swedish west coast and one in Florida (US), representing tropical environment. Some tubes have also had fittings attached to them. The corrosion resistance of the four grades in respective environment has been evaluated after 12-13 months exposure time. The results show that the grades UNS N08904 and S32750 are suitable for temperate climates, while only S32750 is suitable for tropical climates.
Hydraulic and instrumentation (H&I) systems are vital parts and usually massive installations on any land-based or offshore site. They consist of several kilometres of high-quality stainless steel, nickel base or copper alloy tubing with strict dimensional tolerances and hardness values. The tubes are connected using hundreds of fittings, each precision made and expensive. All this sums up to making the H&I systems very costly and it is understandable that plant owners and EPC's want to find the most cost-effective solution. At the same time, safety and operational integrity should not be jeopardised as the H&I systems among other things control safety functions.
When choosing material for H&I installations in inland regions, the commonly used alloy is stainless steel of type 316L (UNS S31603) with matching fittings. Unless there is some specific plant related condition that needs to be considered, 316L will have sufficient corrosion resistance about the environment it is exposed to, thus being a cost-effective, long-term solution. In the case of coastal, marine installations though, the conditions are markedly different. The H&I system will in these cases be exposed to salt spray from the sea, salt deposits form on the tube and coupling surfaces, and seawater can enter inside the fitting and between tube and clamps. For 316L, this environment is generally too tough and will easily cause pitting corrosion on the tubes and crevice corrosion beneath clamps or inside fittings. In worst case, if the metal temperature gets high enough, chloride induced stress corrosion cracking (Cl-SCC) can also develop in this grade which can have catastrophic consequences. Despite this, 316L is often specified also for these harsh environments due to cost related issues or a lack of understanding of the corrosive environment. The expected lifetime of a 316L H&I installation in marine environments is limited though and in a long-term perspective it is not a cost-effective solution.
