Alloy coating extends bearing life in harsh environments

April 20, 2000
A thin zinc-alloy coating that can be applied to production bearings lets them tolerate wet environments and extend bearing life.

A thin zinc-alloy coating that can be applied to production bearings lets them tolerate wet environments and extend bearing life. The coating, called AquaSpexx, is a product specification engineered to increase performance of Timken bearings in applications subject to water-based corrosion damage.

The corrosion protection provided by the coating is classified as sacrificial or galvanic. It is applied directly onto finished components.

Water in rolling mills cools some equipment to maintain their contours, minimize heat checking, and lengthen their service life. On hot mills, high-pressure water jets remove scale from hot steel before rolling and keep surfaces clean between passes.

While water is necessary to maximize roll life and produce high-quality rolled material, it is detrimental to the fatigue life of antifriction bearings. In fact, water corrosion damage is the main cause for work-roll bearings to prematurely reach their useful life. To combat this detrimental effect on bearing life in rolling mills, the zinc-alloy coating applied to bearing raceways protects against water-based damage.

The suitability of AquaSpexx for a given application depends upon the nature of the substance that is causing the corrosion. For environments where water is the corrosive medium, the acidity of the water is a key factor in determining the effectiveness of the coating's performance.

Acidity is commonly measured using the pH scale. Values below 7 (which is neutral) indicate an acidic solution. Values above indicate an alkaline solution. A decrease of one point in pH indicates a tenfold increase in acidity.

Laboratory testing assessed the effect of acidity on coating performance. Two-inch OD AquaSpexx cups were used as test parts. One fresh cup was suspended in each of a series of buffered water-based solutions prepared to pH levels from 2 to 11. Each cup was weighed to the nearest 0.0001 gm before and after 24 hr of exposure in the solutions. The magnitude of the mass change is proportional to the corrosion rate of the coating.

Test results in the accompanying figures indicate that corrosion rates begin to increase as the pH decreases below six. The following application guidelines were developed from the data:
pH < 5.0 The bearing is incompatible with the environment based upon pH
pH ≥ 5.0 and > 6.0 Compatibility requires a laboratory test
pH > 6.0 The bearing is compatible with the environment based on pH

Users must prevent contact with concentrated mineral acids such as sulfuric, hydrochloric, and nitric acids to avoid severe degradation of the coating.

To refurbish the bearings, the coating can be stripped from the cups and reapplied. This extends the protection if normal fatigue or other types of damage has not reduced the useful life past a repairable condition. Any cleaning solution used in servicing the bearings should be alkaline based to minimize coating degradation.

The coated bearings will reportedly lower total operating costs by reducing bearing costs, labor for maintenance, and downtime. The coating is said to eliminate the need for extensive polishing and cleaning to remove water-based damage. Bearings will also last longer because the coating eliminates water-etch staining. Finally, the faster turnaround getting bearings back into service could potentially let mills keep a smaller bearing inventory.

The bearings are adapted for work in steel mills. In December 1998, for instance, Timken installed four sealed roll-neck bearings coated with AquaSpexx in a steel mill's two most brutal locations with excessive water, load, and temperatures. The mill had been using bearings that required quarterly inspection to prevent shutdowns. An inspection seven months after the coated bearings were installed showed no spalling. Moreover, the original seals were still in place. The bearings were reinstalled and have been running smoothly ever since. That's several times the life of the replaced bearings.

This information provided by Timken Inc., Canton, Ohio.

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