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Kaydon white papersStop bearing corrosion before it startsCorrosion is the natural enemy of a rolling element bearing, and a sneaky enemy at that. When chemical reactions from moisture, vapors and acids attack bearing surfaces, the damage they cause often isn't apparent until it's too late. Fortunately, there are many things you can do at the design stage to protect your bearing from these sneak attacks. The first step, as usual, is to completely define the service conditions. The second is to conduct your review of the available techniques of corrosion protection with these requirements in mind. You may find advice from a bearing manufacturer or lubricant producer helpful at this early stage, as they typically have experience in a wide range of applications. Service conditionsThere are many service conditions to consider, but they all fall into five main categories: performance requirements, contaminants, corrosion resistance ratings, frequency of maintenance, and extreme conditions.
1. Performance requirements
These performance criteria are not typically affected by a corrosive operating environment, but you may find it necessary to make a few design modifications to assure that every one of them is met. The nature of the modifications will depend in large part on the nature of the contaminant(s).
2. Identify contaminants It's a good idea to look beyond contaminants and consider the effects of every element in the operating environment. Some are obvious (extreme temperatures and pressure), but others are not. Under some circumstances, for example, dry lubricants will react with water to produce sulfuric acid in the bearing race. Bearings are also exposed to contamination before installation, during manufacturing, shipping and storage. Moisture-proof packaging is a good idea throughout all these stages. In addition, facilities that apply corrosion-resistant coatings tend to have acidic atmospheres, so any uncoated bearings produced there should be protected with a lubricant film.
Salt spray resistance tests Twelve bearing race rings were divided into two sets, each set containing six rings—two each of plain 52100 steel, 440C stainless steel, and 52100 coated with electro-deposited, high-density chromium. Researchers treated one ring of each material with a water-resistant grease meeting the MIL-PRF-23827 specification and left the other ring of each material untreated. The races were suspended in a test chamber and subjected for 96 hours to 5% sodium chloride spray at 95°F. The researchers inspected the rings periodically and recorded the progress of corrosion. The averaged results of the two tests are:
3. Corrosion resistance ratings
4. Frequency of maintenance
5. Extreme environments Corrosion protection optionsOnce all the service conditions have been defined, you can start assessing your bearing protection options and narrow your search accordingly. Lubricants, seals, coatings, and alternate materials are the most common forms.
Lubricants This protection, of course, is only possible when the lubricant film is intact. Should it overheat, dry out, or settle between operating cycles, it cannot be relied upon. Some people try to compensate by over-lubricating, but this simply increases the frictional torque and usually causes the bearing to overheat. Idle equipment should be rotated periodically to maintain the lubricant's protective properties.
Seals
Protective coatings Chromium and nickel plating offer excellent corrosion resistance and are effective in highly corrosive environments, such as acid-bath plants or salt-spray exposures. Baked-on, solid-film lubricants or phosphates of zinc and manganese give good resistance and are suitable for moderately corrosive operating conditions. Preservative oil or black oxide coatings are suitable only for very short-term protection from mildly corrosive environments. They are sometimes used for storing bearings before service. Zinc thermal spray, which gives the metal cathodic protection, offers a longer-term solution. Eventually, however, most platings and coatings begin to separate from the base metal—chipping, flaking, or peeling. One exception: electrodepositing high-density chromium on races of traditional 52100 bearing steel. Independent tests have shown that electro-deposited, high-density chromium—such as the Endurakote® process used by Kaydon Bearings—resists corrosion at least as well as 440C stainless steel. These protective coatings—except for DLC—are applied more often to the bearing rings than the rolling elements, whose shape and comparatively small diameter make it difficult to keep even 0.0001" thick plate within allowable tolerances. For rolling elements, a material change is usually advisable.
Alternate materials ConclusionCorrosion can have a serious impact on bearing life, which in turn affects a machine's performance, productivity, and cost of operation. Design engineers can minimize this impact by considering all of the bearing's operating conditions when setting the initial design criteria, and including one or more corrosion protection alternatives in their specifications. For assistance comparing the options or evaluating them for a particular application, please contact the experienced application engineering staff at Kaydon Bearings. ![]() Download this article
Stop bearing corrosion before it starts |
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