Analysis of Cavity Rust in New Vehicle Development

Introduction

In the process of automotive painting, the whole vehicle anti-corrosion is very important. At present, galvanized sheet and hot-formed sheet are widely used in automotive industry to enhance the anti-corrosion ability of automotive body. Because of cost considerations, some enterprises still use cold-rolled sheet for inner chamber steel plate. Compared with galvanized sheet, cold-rolled sheet has weak anticorrosive ability. This requires that the inner chamber of car body with cold-rolled sheet must be electrophoretically coated and dried to meet certain thickness requirements, so as to achieve long-term anticorrosive effect.

1 Cavity rusting causes analysis

New product development early stage, combined with the same platform experience, in order to avoid the electrophoretic shielding effect and ensure the electrophoretic effect, electrophoretic anti-electromagnetic shielding hole was designed in the corresponding area. Pre-trial production, after on-line vehicle disassembly and verification, the electrophoretic effect of the inner cavity reached the product design and met the anticorrosive requirements.

First rusting reason: production line change. Because of the company's strategic planning needs, the car was transferred to another painting production line. Because of the change of material, production line and electrophoresis equipment, the first time the car crossed the line, it was found that the effect of internal cavity electrophoresis was very poor. Then, on the premise of guaranteeing the strength and function of the car body, the design of the product structure was changed. After many times of verification, the effect met the requirements of anti-corrosion quality.

Reasons for the second rusting: Maintenance and control of production line equipment. The quality of electrophoresis mainly depends on three points: product design, electrophoretic paint and bath parameters, electrophoretic equipment capacity and construction process management. The main reason for the second rusting of this vehicle is the abnormal equipment management.

2 Two-time rusting solution

2.1. The first cavity rusting solution

Because the production line of this model is reconstructed from the old line, there are some limitations in upgrading the equipment capacity of the production line. In order to ensure that the model can be successfully trial-produced according to the project nodes, it is decided to optimize the internal structure of the product after discussion. Specific modification and optimization of the structure of the parts are as follows: There are nine new openings on the three surfaces of the reinforcement plate on the left B column, nine new openings on the three surfaces of the reinforcement plate on the right B column, one new opening on the outer plate of the left A column framework, one new opening on the outer plate of the right A column framework, one new opening on the reinforcement plate after the left A column framework, one new opening on the reinforcement plate after the right A column framework, one new opening on the left threshold reinforcement beam, one new opening on the right threshold reinforcement beam, and the left threshold reinforcement beam. A new sheet metal notch was added to the inner upper plate of the middle pillar and a new sheet metal notch was added to the inner upper plate of the right middle pillar;

After the structural modification of the design, the automobile body (including four doors and two covers) was dissected after pre-treatment, electrophoresis and electrophoresis baking, and the electrophoretic film thickness test data of the left and right inner chambers were up to the standard (standard2.2 Solution to the second cavity rusting

When the second cavity rusting happened, on the basis of summarizing the first rusting rectification, we mainly investigated the sheet metal gap between the outer side plate and the reinforced plate on the B column, the parameters of the pre-treatment electrophoretic bath, electrophoretic permeability and the rate of electrophoretic voltage implementation.

2.2.1 Scaffold clearance check between side periphery panel and reinforcement plate on B column

For the first rusting solution, in order to ensure good electrophoretic effect, electrophoretic shielding holes should be designed for the corresponding position of the new product. However, in order to achieve good intracavity electrophoretic effect, it is not simple to open holes. First of all, from the perspective of stamping, the fewer holes, the better, and the size and spacing of holes should consider sheet metal structure. The electrophoretic effect can be improved by convex platform, optimizing overlap structure and clearance of sheet metal. In addition, the non-welded area between body structural plates should be properly cleared. Generally, the clearance between two layers of sheet metal is more than 4 mm, so as to avoid the problem of poor sharp edge covering ability of electrophoretic paint. 

From the previous electrophoretic anatomy of the vehicle type project, the detection of the sheet metal gap between the side panel of the electrophoretic body and the reinforcement plate on the B-pillar shows that the sheet metal gap between the left and right peripheral panel and the reinforcement plate on the B-pillar is more than 4.0 mm, which can eliminate the bad electrophoresis caused by the sheet metal gap and cause the cavity rusting.

2.2.2 Pretreatment and optimization of electrophoretic parameters

Practice has proved that poor pre-treatment effect of workpiece is the direct cause of poor electrophoretic effect. The effect of pretreatment is affected by the parameters of degreasing, surface adjustment and phosphating tank liquid. For this reason, the sample of each tank liquid of pretreatment is tested. At the same time, the weight of phosphating film and crystalline size were measured by cold rolling plate.

From the above test and analysis data, it can be seen that the parameters of tank fluid are within the technical specifications, the phosphating film is in good condition, and the cavity rusting caused by poor pretreatment can be eliminated. The electrophoretic bath parameters and electrophoretic construction parameters are undoubtedly the important factors leading to poor electrophoretic quality. In the electrophoresis process, the solid content, conductivity, temperature, pH, electrophoretic permeability and other factors directly affect the anti-corrosion ability of electrophoresis. For this reason, the electrophoretic tank fluid samples are tested.