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Electrocorrosion

Fretting wear arising from small-amplitude sliding and rolling at bearing contact surfaces manifests as fretting (wear) corrosion and pseudo‑indentations, respectively; these two wear mechanisms are distinct.

Failure Mode: Electrochemical Corrosion

Keywords: Overcurrent corrosion

Sequential removal of materials High current: spark Localized heating in an extremely short time Melting/Welding The pit depth reaches 0.5 mm.

Principle

1. When current passes through the bearing—specifically, when it flows from one raceway to the other via the rolling elements—damage occurs (Figure 1).

2. During the contact process, similar to arc welding, a high current density is generated over a very small contact area (Figure 2).
3. The steel is heated to temperatures ranging from tempering to melting, resulting in discoloration zones of varying appearance and size. Within these zones, the material has undergone tempering, followed by hardening or melting; spherical particle accumulations also form at the interfaces where the material melts and fractures (Figure 3).
4. Excess material in the contact area can lead to wear or surface‑originated spalling (Figure 4).

Phenomenon

Spherical particle accumulation on the raceways and rings. Occasionally, zigzag‑shaped burns appear in the raceways of ball bearings. Localized burns on the raceways and rolling elements.

Excessive current flowing through the rollers of a spherical roller bearing results in significant pitting, which is markedly amplified. The extent of the damage, including the pitting and the melted and solidified material, can be clearly observed, as shown in the figure below:

1. Configure and troubleshoot the circuit to prevent current from passing through the bearings.
2. Use insulated bearings.

Application Cases

1. Air Compressor Case Study
20190725 Application Case: A customer in the air compressor industry, using an oil-free 550W diaphragm pump, reported that after less than two months of operation, unusual noises began to occur. The equipment was fitted with two sets of 6203-2RS bearings.
According to the inspection conducted by our company’s technical personnel, all dimensional measurements are within normal limits; however, the clearance has been adjusted from C3 to C0, and the noise level exceeds the specified threshold. The detailed analysis is as follows:
1. The bearing returned from the customer’s site is shown in the figure: its exterior exhibits no obvious signs of installation or wear; however, when rotated, it feels noticeably stiff and binds axially, suggesting preliminary damage to the raceways.

2. After prying open the bearing cover, the internal grease was in good condition, showing no discoloration and retaining its lubricating performance.

3. Under the microscopic examination of bearing #1, pitting is observed across the raceway surface, indicating micro‑current corrosion; localized burn marks are present on the raceway, arranged in a zigzag pattern.

4. Under the microscopic image of bearing #2, significant pitting is observed on the raceway surface, also attributable to electrical erosion; however, the current passing through the contact interface is relatively higher than that in bearing #1.

Draw a conclusion Due to equipment leakage (the air compressor lacks a leakage protector), current passes through the bearing, creating a potential difference between the outer and inner rings. This causes the lubricant film to break down, resulting in bearing damage and abnormal noise.

Motor Case

20190906 Application Case: A customer in the motor industry reported that their end‑user observed an abnormal noise immediately upon starting the motor. The motor is equipped with two sets of 6203‑2RS bearings.
According to testing by our company’s technical personnel, all dimensional measurements are within specifications; only the noise level exceeds the standard. The detailed analysis is as follows:
1. The bearing returned from the customer’s site is shown in the figure: its exterior exhibits no obvious signs of installation or wear; when rotated, it feels noticeably stiff, and the elevated noise level further suggests that the raceway has been damaged.
2. After prying open the bearing cover, a distinct, irritating odor was detected; the grease near the outer race had discolored (turned yellow), though it had not yet completely lost its lubricating performance.

3. Internal Conditions

Draw a conclusion Due to equipment leakage, current passes through the bearing, creating a potential difference between the outer and inner rings. This causes breakdown of the lubricant film, leading to electrical erosion. Electrical erosion, along with impurities and raceway defects, results in fretting wear during radial micro‑motion operation.

The main causes of bearing false brinelling are:

1. Vibrations and oscillations of the bearing during transportation and when it is stationary;
2. Oscillatory motion with small amplitude;
3. Poor lubrication.

Pump Case Study

20190906 Application Case: A customer in the pump industry reported that a submersible well pump had seized and failed. The motor was equipped with 6203-2RS bearings.
According to testing by our company’s technical personnel, all dimensional measurements are within specifications; only the noise level exceeds the standard. The detailed analysis is as follows:
1. The bearing returned from the customer’s site is shown in the figure: its exterior exhibits no obvious signs of installation or wear; when rotated, it feels noticeably stiff, and the elevated noise level further suggests that the raceway has been damaged.
2. After prying open the bearing cover, the grease near the inner and outer rings has discolored (turned black), but it has not yet completely lost its lubricating performance.

3. The inner raceway exhibits overall wear. Wear marks are evenly distributed along the raceway, with a higher concentration of electrical erosion pits at the centers of these marks; extending outward from these pits is fretting wear.

4. The outer raceway has turned uniformly yellow. Within the raceway, wear marks are evenly spaced; at the centers of these marks, numerous electrical erosion pits are observed, and micro‑slip wear radiates outward from these pits.

5. The steel ball’s surface has turned uniformly yellow. There are severe surface scratches, and obvious pitting caused by electrical erosion is present.

Draw a conclusion Based on the analysis of the failure symptoms, it is determined that equipment leakage current passes through the bearing, creating a potential difference between the outer and inner rings. This leads to breakdown of the lubricant film and subsequent electrical corrosion. The resulting corrosion products and raceway defects, combined with radial fretting during operation, give rise to fretting wear.

Keywords: current leakage corrosion

Low current intensity Near the pit location Chip grooves parallel to the rolling axis on the raceway and rollers are developed. Turns dark gray

Failure mechanism

Initially, the surface is damaged by shallow pits that are closely interconnected; compared with damage caused by excessive current, these pits have smaller diameters. This phenomenon can occur even at relatively low current levels. (Magnified images: 500×, 5,000×)

Grooves are found on the raceways of both the inner and outer rings, not on the steel balls. The extent of the damage depends on numerous factors: current magnitude, duration, bearing load, speed, and lubricant.

A prime example illustrating the damage caused by current leakage in cylindrical roller bearings. Both the raceways and the rollers exhibit visible marks. Note the grease: in the early stages of this failure mode, the grease carbonizes, eventually leading to surface damage, spalling, and even seizure.

Under the microscope, we can observe numerous tiny pits on the surface.

Further magnification reveals the material’s microstructural changes. The white region has re‑hardened, typically attaining a hardness of 66 to 68 HRC. Beneath it lies a black layer that has undergone thermal annealing and is softer than the surrounding bearing steel (56–57 HRC).

Motor Case

20200526 Application Case: A customer in the motor industry used a 6201 bearing from a certain imported brand in an oven motor. The end user reported abnormal noise and bearing disintegration, resulting in failure.
Following analysis by our company’s technical personnel, the results are as follows:
1. The inner raceway exhibits numerous closely spaced elongated depressions, appearing dark gray; the running track shows varying widths, gradually narrowing toward one side.

2. The outer raceway exhibits numerous closely spaced, elongated depressions, appearing dark gray; the running track gradually narrows toward one side until it disappears.

3. The rolling elements exhibit severe surface wear, have turned white, and show numerous pitting defects.

Draw a conclusion Bearing failure due to erosion caused by electrical current leakage. Causes: a) Potential difference between the outer and inner rings; b) Equipment capable of generating high-frequency signals, or printed circuit boards; c) High-frequency potential differences arising in the surrounding environment.

Remedial measures

a. Configure the circuit to prevent current from passing through the bearing.

b. Insulate the bearing

Fretting wear corrosion

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