316 types stainless steel are used widely in marine application, but their corrosion resistance in contact with seawater is limited and they cannot be considered ‘corrosion proof’ under all situations. They are susceptible to localized attack mechanisms, principally crevice and pitting corrosion. This limits the scope for the use of these steels in seawater contact.
Type 304, and more especially the free machining 303 types, should not be considered suitable for seawater service. Sulphide inclusions outcropping on the surface of the 303 type are preferential pitting corrosion sites.
Factors governing the corrosion resistance of 316 types in seawater
The factors governing the corrosion resistance and hence suitability of the 316 types has been well documented by many workers in these fields of research.
These factors work together and include
- Water quality
- Cathodic protection
The chloride levels can vary depending on the location and influence of tides. The levels encountered in even ‘brackish’ waters are above those where crevice corrosion can be expected to be a corrosion hazard. Intermittent exposure, for example in tidal zones, has been noted as less of a corrosion risk. This may be due to the fact that the steel surfaces are effectively ‘washed’ by the changes in water levels. Water evaporation effects could however increase the corrosion risks in splash zones, if the chlorides concentrate in a damp or wet environment.
It is important not to let seawater stand in contact with the steel unnecessarily. Horizontal 316 tube sections handling seawater have been noted to fail by pitting after only short periods.
Free draining surfaces and the avoidance of horizontal tube runs are important to the successful use of 316 in contact with seawater. If tubing systems are hydro-tested using seawater, this must be drained and flushed immediately after the test period. Failure to do this has resulted in corrosion to 316 systems.
Water flow rates
‘High’ flow rates are preferable (ie over 1 metre / second). Slow moving water can encourage biofouling, which can then result in shielding or crevice corrosion. Stagnant seawater conditions must be avoided. Increases in flow rates reduce the risk of corrosion and so applications such as pumps, can be successful applications for 316 types in seawater handling.
The crevice corrosion risk increases with temperature. Contact with heated seawater is not advisable. Ambient temperatures in northern European waters, as a guide, are around the maximum that a 316 should be expected to cope with, even if other conditions are favorable.
Stress corrosion cracking is not usually a concern in the temperatures that the 316 would be used at. (Higher temperatures would probably result in crevice and pitting corrosion anyway)
Water oxygen levels (deaeration)
Stainless steel rely on a source of oxygen to maintain their passive condition. Aerated seawater however can be more corrosive than de-aerated seawater.
It has been found that very low levels of oxygen, such as those found at sea depths of around 200 metres, make seawater less aggressive. This is associated with the slowing down of pitting corrosion rates.
Cathodic protection can be applied ie electrically or derived from contact with less ‘noble’ metals, including carbon steel and aluminium. Direct contact with these metals can help improve the resistance of the 316 types of stainless steel, at the expense of the other metal.
Although the stainless steel can benefit, there may be a concern that the overall durability of a fabrication involving such combinations could be compromised.
‘Engineered’ crevices (surface finish and post fabrication cleaning)
Crevice and the closely related pitting corrosion mechanisms are the forms of local attack that are normally responsible for the failure of the 316 types in seawater service.
Any form of crevices must be avoided.
These can occur through
- Design geometry (sharp corners or grooves)
- Flanged joints with gaskets
- Mechanical fastening systems
Intergranular corrosion has been detected on laboratory sensitized (heat-treated) 316 when subsequently exposure in seawater. The use of the low carbon 316L types such as grades 1.4404 or 1.4432 should avoid this additional corrosion risk in welded structures.
The surface finish weld quality and finishing of the steel can be important factors in the successful use of 316 types in seawater service applications.
These may be more important issues than factors such as the actual chloride concentration. Smooth, clean finishes and well-finished welded joints contribute to the corrosion resistance of the steel.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)