Analysis of the Corrosion of Fasteners Dealed with Dacromet and Solutions

Analysis of the Corrosion of Fasteners Dealed with Dacromet and Solutions

Abstract: Taking the problem of fastener corrosion of urban rail vehicles as the research object, the causes of corrosion of lock washer after Dacromet treatment were analyzed experimentally and theoretically, and specific solutions were proposed.

1. Proposal of the problem

Most of the welding structures of urban rail vehicles are mainly made of aluminum alloy. In order to meet the anti-corrosion requirements, the fastening bolts used to install the vehicle equipment and its components are mostly made of stainless steel, but to ensure the anti-corrosion and anti-rust performance of the fasteners and reduce the overall Cost, a large number of Dacromet-treated lock washers are used in conjunction with stainless steel bolts, such as lock washers for components such as pantographs, lightning rods, and air conditioners on the roof of urban rail vehicles. The lock washer is made of spring steel and the surface is treated with Dacromet. It is used with A4-80 stainless steel bolts and nuts. However, after using it for a period of time, it was found that the lock washer used on the outside of the vehicle, such as the lock washer at the shock-absorbing mounting bolt of the roof air conditioner, was rusted, and the gasket which was fastened in the interior and the underframe was not found to be rusted. Happening.

2 , preliminary analysis of the cause of corrosion of washers in Dacromet

The cause of metal corrosion is generally mainly due to electrochemical corrosion or air erosion.

(1) Dacromet coating does not meet the requirements, Dacromet surface coating has defects such as foaming, cracking, pinhole and leak coating, which cannot be effectively analyzed. Dacromet surface treated lock washer appears. Corrosion protection, causing oxidative corrosion.

(2) Dacromet-treated lock washers are used in combination with stainless steel bolts. Due to the different corrosion potentials of the two metal materials, the spring steel washers with lower potentials are more likely to dissolve and rust.

3. Test verification and analysis

In order to verify the above two reasons that may cause the gasket to rust, it is mainly tested and analyzed from material analysis, low-profile inspection and salt spray comparison test.

3.1 Test verification

3.1.1 Lock washer material inspection

Remove the two DAC320 lock washers SKM12 from the application site, remove the Dacromet coating on the surface of the substrate by grinding, and use the spectrometer to detect the chemical composition of the substrate. The test results are shown in Table 1, which meets GB/T1222-2007. 65Mn material requirements.

3.1.2 lock washer low magnification test

Test method: 3 new DAC320 lock washers SKM12 were taken immediately; after a simple surface cleaning (a pickling, a rinse of water, an alkali wash); it was observed by a naked eye and a magnifying glass magnified 40 times. It was observed that the surface of the gasket was smooth and silvery gray, and no obvious cracks, blistering and the like were found on the surface of the gasket.

3.1.3 lock washer salt spray comparison test

Test method: 4 new DAC320 locking pads 圏SKM12 are randomly selected, 2 A4-70 bolts and A4-80 nuts are used together with the washer; the washers, bolts and nuts are divided into 2 groups, the first group is 2 The gasket used separately, the second group is the structure of the washer and the bolt and the nut; the two groups of samples are placed in a salt spray tester with a concentration of 5% NaCl solution, the test temperature is 35T, and the intermittent spray is used (spraying for 5 minutes) , stop for 10 min); observe the test process at regular intervals until the gasket shows obvious rust.

test results:

(1) The washer and the separate washer in the bolt washer assembly before the start of the test, the surface is silver-gray, no rust.
(2) After 24 hours of testing, it was observed that a small amount of rust appeared in the gasket in the bolt washer assembly, and the separate gasket did not change before the test, as shown in Fig. 1.
(3) After 48 hours of testing, it was observed that the gasket in the bolt washer assembly showed obvious rust spots, and the single gasket did not change before the test.
(4) After 72 hours of the test, it was found that the gasket in the bolt washer assembly was severely corroded, and the separate gasket was basically unchanged from the pre-test, as shown in Fig. 2.

3.2 Analysis of test results

The results of the comparative salt spray test of the two sets of gaskets show that:
(1) The gasket treated with Dacromet alone was tested for good corrosion resistance, and no corrosion occurred after 72 h salt spray test;
(2) When used in combination with stainless steel bolts, its corrosion resistance is significantly reduced, and rusting begins to occur after the 24h salt spray test.

Analysis of the Corrosion of Fasteners Dealed with Dacromet and Solutions

After analysis, the potential of Zn in the Dacromet coating is -0.762, while the potential of stainless steel A4 (0Cr17Ni14Mo2) is positive. There is a large potential difference between the two, and galvanic corrosion occurs in the aqueous medium. The two kinds of metals constitute a macro battery, which generates a galvanic current, which accelerates the dissolution of the lower potential Dacromet coating (Zn=Zn2++2e). When the coating dissolves to a certain extent, the substrate of the lock washer is exposed to moist air, and the Dacromet coating loses its anti-corrosion function, causing rust of the spring steel (lock washer).

In summary, the reason why the surface of the Dacromet-treated lock washer is rusted during use is that the potential difference between the carbon steel Dacromet coating and the stainless steel is not taken into account during the fastener selection process. Large, coupled with the harsh environment, the presence of aqueous media, causing accelerated corrosion.

4, the solution

Considering the harsh environment of the lock washer (outdoor), according to the principle of galvanic corrosion, the same metal should be used in the same mechanism, or two metals with the potential as close as possible, and separated by insulation to avoid Galvanic corrosion occurs. Therefore, it was determined to replace the spring steel washer with a lock washer of stainless steel A4.

Article source: "Motorcycle Technology" Zhu Xiangdong Chen Xuehai Fu Wenbing
(CSR Zhuzhou Electric Locomotive Co., Ltd., Zhuzhou, Hunan 412001)

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