Corrosion test, analysis, mechanism and prevention measures.

Abstract: through the corrosion Comparison test, it is determined that the corrosion of the sealed ball bearing for motor is caused by the acidic substances volatilized from the insulating varnish impregnated on the motor stator. Further, the rust and the extract are analyzed by X-ray energy spectrum, X-ray diffraction and ion chromatography, and the volatile substance is determined to be formic acid (HCOOH), and the corresponding prevention and control measures are put forward.

Key words: motor bearing; rust; formic acid (HCOOH)

The sealed deep groove ball bearing for motor often has poor starting and increased noise in the early stage of use. After removing the seal, cleaning and drying, it is observed under the microscope that the bearing has been rusted, and has the following characteristics and laws: (1) rust often occurs on the surface of the ferrule and steel ball, and rust spots sometimes appear on the outer surface of the cage ball pocket and the assembly chamfer of the ferrule. (2) When the motor is stored in a warehouse with high humidity and temperature, it will happen in about a week. (3) There is massive rust on the surface near the rotor of the motor. In serious cases, there is black dot rust on the outer surface of the bearing. (4) The color of grease changes, and there is a pungent smell of paint. (5) For particularly serious cases, rust spots are loose and porous, and are easy to fall off automatically. This corrosion phenomenon of motor bearings is very similar to the paint rust reported abroad.

According to the market feedback, the above rust mostly occurs in the sealed ball bearings for small and medium-sized motors, which are more serious and frequent in the south, and bring huge economic losses to the bearing and motor production enterprises at the same time. Therefore, it is necessary to find out the root causes and take measures to prevent it.

1. Corrosion test

The motor bearing is in the environment of insulating varnish, and the substances that can be in direct contact with the bearing ring are: antirust oil, grease. But generally speaking, anti-rust oil and grease will not cause bearing rust. The possible reasons for bearing corrosion in this environment are: the insulating varnish impregnated in the motor stator volatilizes gaseous substances, which makes the coated antirust oil unable to effectively prevent rust, or the lubricating grease absorbs this gaseous substance and causes physical and chemical reactions, resulting in grease discoloration and bearing corrosion. In order to confirm this view and find out the reasons, the following comparative tests were carried out: rust tests were carried out on different antirust oils, different greases, different varnish types and different bearings at home and abroad. The test conditions are: hang the test bearing in 1038 varnish atmosphere at 60 ℃. In a constant damp heat test chamber with a humidity of 70% RH. Observe all tested bearings after 1 week.

1.1 Comparison of corrosion tests of different rust preventive oils

Four different antirust oils are selected. In order to prevent the cross influence of antirust oil and other substances, the test bearing channel is only coated with antirust oil without any grease. One week later, the samples were observed, and the results showed that the bearings coated with these four kinds of antirust oils were seriously rusted.

1.2 Comparison of corrosion tests of different greases

Two lubricating greases (both lithium grease) with different base oils were selected for the test. Similarly, in order to prevent the cross influence of grease and other substances, the test bearing is only injected with grease and not coated with any antirust oil. Results after injecting these two greases, serious corrosion also occurred in the environment set by the test.

1.3 Comparison of bearing corrosion tests abroad

6201 closed bearings produced by two foreign companies were selected for the test. After the test, it was found that the bearings were also seriously rusted under the environment set by the test.

1.4 Comparison of different varnish tests

The amino alkyd varnish used in the above tests is 1038. In order to investigate the influence of different varnishes on corrosion, another epoxy resin varnish 831 was selected, and the bearing was not corroded after 1 month of bearing test in epoxy varnish atmosphere (other conditions are the same as above).

It can be seen from the above results that under the environment set by the test, when amino alkyd paint is used, no matter which kind of antirust oil is applied, which kind of grease is injected, or the bearings of foreign famous brand companies have serious corrosion. When the epoxy varnish was used for the test, the tested bearing still did not rust after 1 month of the test. Therefore, it is proved that the varnish with the brand of 1038 volatilizes strong acidic gaseous substances, so that the coating of general antirust oil and grease can-not effectively prevent the corrosion of bearings, so the insulating varnish is the main cause of bearing corrosion.

2. Corrosion analysis

In order to find out what kind of substance volatilized from insulating varnish leads to bearing corrosion, the rust powder and varnish volatile extract were analyzed by X-ray diffraction and ion chromatography. The corrosion powder sample is obtained from the above corrosion test: in order to obtain the extract of the volatile matter of the varnish, the gas volatilized from the varnish is introduced into the NaOH solution.

2.1 Rust powder analysis

Carry out energy spectrum analysis on the rust on the bearing surface: there are Fe, Cr, C, O and other elements. However, no abnormal elements were found.

2.2 X-ray diffraction and ion chromatography analysis

Because the insulating varnish volatilizes gaseous substances, it belongs to gaseous corrosion. The gas phase substance is obtained by liquid phase extraction method, and the PH measured by PH test paper is ≤ 6.0, and the extracted substance is acidic. Therefore, in the test, NaOH solution can be added to the extraction solution to collect the volatile substances from the varnish, and the collected substances can be concentrated and evaporated to obtain a small amount of precipitates.

First, the precipitates were analyzed by X-ray diffraction and compared with the standard spectrum to confirm that the precipitates contained sodium formate (Na COOH). Since formic acid can dissociate into formate anion (COOH) and proton (H+) in aqueous solution, and formate is not volatile, it can be inferred that the acid extracted is formic acid.

Secondly, according to the test, the bearing corrosion is very serious, so it can be suspected that the volatile matter is a kind of strong acid. The suspected objects are inorganic acids, such as hydrochloric acid, sulfuric acid, etc; Organic acids, such as benzenesulfonic acid, formic acid and acetic acid. Therefore, the extraction solution was analyzed by ion chromatography, and the known aqueous solution of formic acid was used as the reference sample. The extract contains a large amount of formate (COOH-). There are also CL- and SO₄-, whose contents are in the order of 2ppm.

The above analysis shows that the extract contains formate ions, and formate is difficult to volatilize; In addition, the extract measured with pH test paper is obviously acidic, so it shows that the volatile matter of insulating varnish is formic acid. Referring to relevant data, we know that long branched chain fatty acids are needed to prevent the synthesis of alkyd resin, otherwise the reaction will be difficult to control and gelation will often occur. At this time, unit acids can be added as chain terminators to control the length of the chain. The curing process of amino alkyd resin paint is a complex process dominated by copolycondensation with other side reactions such as self- condensation. Among them, the ether exchange reaction plays a major role, and the ether exchange reaction usually requires acid as a catalyst. Therefore, when designing fast drying amino alkyd resin paint, the introduction of unit acid has two functions: one is to control the esterification reaction as a chain terminator. Second, the unreacted carboxyl group catalyzes and promotes the crosslinking, improving the curing reaction speed. Therefore, from the reaction principle of amino alkyd resin paint, it is completely established that formic acid exists in volatile matter.

3. Corrosion mechanism

From the failure form of the bearing after the test, the ferrule is rusted, and the grease has also changed color and has a pungent odor. Through the previous analysis, it can be seen that these changes of the ferrule and grease are caused by the formic acid volatilized from the insulating varnish. In a humid environment, the grease is easy to absorb water, so the formic acid volatilized from the insulating paint is very easy to be absorbed by the grease, while the absorbed formic acid aqueous solution is easy to dissociate h+, and a large amount of dissociated H+ and Fe undergo chemical and electrochemical corrosion:

Therefore, the test bearing will rust in a very short time.

In addition, the grease is easy to decompose in acidic environment, resulting in grease failure and further corrosion of bearing surface.

4. Conclusion and prevention measures

(1) The phenomenon of poor starting and increased noise of sealed deep groove ball bearing for motor in the early stage of use is caused by the corrosion of bearing parts, especially the surface of ferrule and steel ball.

(2) Corrosion is caused by the reaction of volatile formic acid with bearing parts and lubricating grease after the motor stator is impregnated with insulating varnish and the insulating varnish is not completely dried or not dried according to the process provided by the varnish manufacturer.

(3) The variety, quantity and dosage of rust prevention oil and grease replacement have no obvious effect on corrosion prevention. The corrosion of sealed motor bearings has a direct and fundamental relationship with the insulating varnish painted on the motor rotor. Using epoxy resin varnish or other solvent-free insulating varnish, increasing the drying time of the varnish and prolonging the holding time at a lower temperature can effectively prevent the occurrence of paint rust.

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