Corrosion protection for bearings - the many alternatives

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Dr Steve Lacey, Engineering Manager at Schaeffler UK, explains the importance of selecting the correct coating or corrosion-resistant material for rolling bearings destined to operate in harsh environments.

Corrosion protection for bearings - the many alternativesBy investing in a brand new development centre for new surface coating of bearing components and by continuing to invest in R&D activities, the Schaeffler Group can offer customers a comprehensive range of special bearing materials and coatings, designed to improve performance and protect the bearings from corrosion, wear, friction and slippage in every application.

The new coatings development centre, opened earlier in 2007 at the Schaeffler Group’s headquarters in Herzogenaurach, Germany, will research and develop new processes and combinations of unique materials and bearing coatings technologies.

While Schaeffler’s standard INA and FAG branded bearings guarantee the customer excellent performance and a long service life, in some harsh operating environments or for special applications a standard bearing material or coating may need to be upgraded in order to prevent wear or corrosion of the bearing.

Coatings are applied to the surfaces of rolling bearing components without forming a material bond, for example by diffusion between the coating and base material. In many cases it is sufficient to coat only part of the bearing or one of the parts that come into rolling contact. Schaeffler can advise customers when a coating is appropriate and which type should be used in certain applications.

Corrosion-resistant materials

As well as offering special corrosion-resistant coatings such as

Corrotect, Protect A and Protect B, Schaeffler can also provide a range of corrosion-resistant materials for rolling bearings that operate in corrosive environments.

While standard steel bearing materials provide satisfactory corrosion resistance in many applications, for higher performance requirements, highly corrosion-resistant, nitrogen alloyed martensitic HNS (high-nitrogen steel) grades – such as Schaeffler’s Cronidur and recently developed Cronitect steels – are also available.

Cronitect is a high-grade martensitic hardening steel, developed for rolling bearings, which offers high resistance to corrosion under extreme conditions - for example, in dry running applications or when the bearings are in contact with aggressive media such as water, acids and cleaning agents. In contrast to Cronidur, Cronitect has nitrogen introduced into the structure by means of a surface layer hardening process.

Based on the consistent refinement of high-grade NIROSteels, Cronitect achieves a very high hardness through its chemical composition, in combination with a new thermo-chemical surface layer treatment process. The material is therefore able to withstand extreme conditions, including salt spray testing in accordance with DIN 50021 SS without any problems, even after 600 hours. Cronitect’s resistance to corrosion is many times better than that of conventional NIROSteels. Rolling bearings that use Cronitect come in either a sealed or non-sealed version.

The name Cronitect is derived from the chemical composition of the material and from its special heat treatment process. The ‘tect’ suffix is found in other Schaeffler coating products and refers to the material’s excellent corrosion protection.

Both Cronidur and Cronitect claimed to be superior to conventional corrosion-resistant steels for rolling bearings in terms of corrosion resistance, wear resistance and fatigue strength. This means they have very high corrosion resistance against aqueous media and heavily diluted acids, alkalines or salt solutions. Potential applications can be found in clean rooms, electronic component manufacturing plants, pharmaceuticals and food production.

Ceramics and plastics

Other special materials include ceramic rolling bearing components, which provide excellent characteristics for certain applications. Silicon nitride balls, for example, are light and offer a longer operating life than standard steel ball elements.

High-performance plastics such as PEEK can also be used for bearing components operating in corrosive environments. PEEK is resistant to high temperatures and, depending on the load, is suitable for operating temperatures of up to 250degC. The wear resistance is strongly influenced by the filler materials and is significantly better than that of glass-reinforced PA66, which is often used for bearing cages.

Schaeffler offers three special coatings that improve the corrosion-resistance and/or wear resistance of the surface.

Corrotect is a relatively low cost, 0.5-5um thick zinc alloy coating with cathodic protection, which is effective against condensation, rainwater, contaminated water and weak alkaline and weak acidic cleaning agents. Under load, the coating is compacted into the surface roughness profile and is partly worn away. The chromate coating and the passivation increase anti-corrosion protection and contribute to the optical appearance of the component.

Corrotect is suitable for small bearings and bearing mating parts that need to have a greater resistance to corrosion - for example, drawn-cup needle roller bearings with open ends, and thin-walled components produced in large numbers.

Protect A is a pure, matt grey coloured chromium coating with a pearly surface structure. The coating is applied by electroplating, so the coated parts retain full dimensional stability. Lubricant is retained between the 'pearls', so effective anti-wear protection is achieved even under mixed friction or slippage conditions. During running-in, the rolling elements and seals burnish the surface, which leads to reduced friction coefficients. Protect A is resistant to various chlorides, oils, sulphur compounds, chlorine compounds and weak acidic media.

Protect B is suitable for use in aggressive atmospheres at high temperatures. It comprises two layers: a thin chromium coating covered by a chromium mixed oxide layer, the latter providing corrosion-resistance by supporting the lubricant. The coating offers high anti-wear protection together with high anti-corrosion protection. Protect B is also suitable for applications in which continuous lubrication of the bearing cannot be guaranteed.

Zinc-based coatings

For corrosion and fretting corrosion, Schaeffler has developed a range of coatings to protect bearings, including zinc iron coatings for rolling mill applications, zinc nickel for bearings used in automotive engineering and zinc phosphate for applications exposed to salt or sea water.

Black oxidising coatings are also available, which are suitable for rollers and rings for bearings in wind turbines and rail traction motor bearings. These coatings are useful for applications in which there is a minor risk of slippage. Thin dense chromium coatings (TDC) also offer effective wear protection in mixed-friction conditions for small ball and roller bearings and are suitable for raceways on high-precision bearings and spindle bearings.

A copper coating is also available for emergency lubrication on bearing cages, while silver coatings are used for the same purpose on aerospace bearings. Titanium nitride offers wear protection in jet engine bearings.

Electrical insulation

As many rolling bearings are used in electric motors, the bearings must be electrically insulated to prevent passage of electric current. For such applications, the outside diameters and the faces, or alternatively the bore and the faces, are coated with aluminium oxide. Schaeffler offers two different coating thicknesses: thick insulating coating with a high resistance to prevent current passage even in moist environments; or a thin insulating coating with a low resistance to current passage. This coating has sturdier edges due to the lower coating thickness.

A thick coating of aluminium oxide is suitable for applications involving three-phase motors and traction motors, while a thin coating is suitable for applications involving traction motor bearings and tapered bearings for axle bones.

Schaeffler’s corrosion-resistant coatings and materials can be used throughout the company’s product range.

For applications in the food industry, for example, Schaeffler’s range of housed bearing units (flanged, take-up and plummer block designs) can be protected with various coatings. The housings and insert bearings can be protected using Corrotect and the housings themselves can also be made from a corrosion-resistant (stainless) sheet steel or plastic (glass fibre reinforced PBT).

Plastic housings are highly resistant to moisture, UV radiation, bacterial and fungal attacks, as well as many chemical media. These bearing units are used predominantly as locating bearings, but are also suitable as non-locating bearings under low loads and speeds.

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