INDUSTRY NEWS Combating Corrosion at the Design Phase
In his follow up article to ‘Is all bearing surface damage troublesome?’ Chris Johnson, Managing Director of bearing specialist
SMB Bearings, explains that while not all bearing surface damage should cause alarm, offshore facility managers should weigh up their design requirements and can choose to combat corrosion at the design stage. Here are three bearing corrosion control methods to consider.
Material Choice
Ensuring the right equipment is selected is the first step to corrosion control — and this is imperative for both large-scale machinery and small components, such as bearings.
Stainless steel is the most obvious material choice for corrosion resistance. 440 grade stainless steel bearings have good resistance in damp environments but have poor resistance to salt water and many stronger chemicals, so for harsh offshore environments 316 stainless steel may be considered. However, as 316 stainless steel cannot be thermally hardened, these bearings are only suitable for low load and low speed applications. Their corrosion
resistance is best when there is an ample supply of oxygen so these bearings are mainly used above the waterline, in flowing seawater or where the bearings can be washed down after submersion in seawater.
An alternative material option is ceramic. Full ceramic bearings made from zirconia or silicon nitride with PEEK cages can offer even higher levels of corrosion resistance and are often used fully submerged. Similarly, plastic bearings, with 316 stainless steel or glass balls, provide very good resistance to corrosion. Like 316 grade bearings, these should only be used in low load and low precision applications.
Another level of armour against corrosion, is a protective coating. Chromium and nickel plating offer good corrosion resistance in highly corrosive environments. However, coatings will eventually separate from the bearing and need continual maintenance. This isn’t the most practical option for offshore applications.
Lubricants Surface roughness and lubrication quality are extremely important influencing factors as to whether surface distress will occur or not. Opting for the correct lubricant matters. In an environment where superficial corrosion may occur on the outside of the bearing, it should not be allowed to occur on the inside.
Sealed bearings with waterproof greases that contain corrosion inhibitors protect the internal surfaces of the bearing and can be matched to the specific offshore application environment. Full ceramic bearings are mostly specified without lubrication but can be lubricated with waterproof grease for extended life.
Seals In harsh environments, contamination protection is of utmost importance, so opting for a contact seal is favourable to ensure contaminants do not enter the bearing. For equipment that may be exposed to moisture, a contact seal will also offer increased water resistance. This will stop grease washing out of the bearing, allowing it to do its job in lubricating and protecting the internal surfaces of the bearing. An alternative option is a metal shield but this offers greatly reduced protection against moisture.
By considering a bearing’s full environmental operating conditions, design engineers can weigh up whether opting for a corrosion control design feature will be the most cost effective, increase the bearing’s lifespan and elevate a machine’s performance.
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www.sosmagazine.biz October 2020
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