Research On Anti-Rust Technology Of Automotive Die-Cast Aluminum Alloy Parts

1 Introduction

       With the development of automobile lightweight technology, lightweight materials are increasingly used in car bodies. For example, on the premise of ensuring performance, non-metals are used to replace metals, and low-density metals are used to replace high-density metals. The material of many parts of the car has been replaced from cast iron to die-cast aluminum alloy, which reduces the quality of the vehicle, thereby reducing fuel consumption and emissions. However, die-cast aluminum alloy parts are prone to corrosion under natural conditions.

       Some die-cast aluminum alloy parts of the body are not protected by paint coatings. For example, some aluminum parts inside the engine compartment and on the chassis will corrode before the vehicle is sold. Although it does not affect its mechanical properties, it seriously affects the appearance quality and leads to consumer complaints. Even claim. At present, there is little systematic in-depth research on the corrosion problem of aluminum castings, and the lack of suitable anti-rust technology has been plaguing OEMs and aluminum casting parts suppliers. Based on the corrosion phenomenon, this article systematically analyzes the causes of corrosion, and formulates anti-rust measures from the entire process of die-cast aluminum alloy parts research and development, manufacturing, storage and transportation, assembly assembly, vehicle assembly, storage and transportation, and forms operating documents and corporate standards. After the implementation of various anti-rust measures, complaints caused by corrosion of die-cast aluminum alloy parts were eliminated.

2. Corrosion mechanism

       There are three main reasons for corrosion of aluminum castings:

       a. After the aluminum parts are shot blasted, the oxide film formed on the surface is not dense and cannot protect the base material;

       b. Aluminum element and Al2O3 are both amphoteric substances and can react chemically with acids and bases (when reacting with acid, Al+HCl→AlCl3, Al2O3+6HCl→2AlCl3+3H2O; when reacting with alkali, Al+KOH+H2O→ KAlO2, KAl2O3+2KOH→2KAlO2+H2O);

       c. Aluminum parts will accelerate corrosion when exposed to water or dust. Every particle of dust is a corrosion core. The dust structure is loose and porous. It is easy to absorb moisture, sulfide, chloride, etc. in the air under high humidity in summer and condensation in winter. Acidic substances are formed, causing corrosion of aluminum parts.

3. Analysis of the true cause of corrosion

       Before formulating a rust prevention plan, you must first understand the current manufacturing, storage, transportation, and assembly processes of die-cast aluminum alloy parts (Figure 1), and analyze the corrosion risks in the entire process.

       As can be seen from Figure 1, no anti-rust treatment is performed on the cast aluminum parts during the entire manufacturing process. There is a risk of corrosion in single-piece cleaning, packaging, single-piece storage and transportation, assembly storage and transportation, complete vehicle storage and transportation, and in-store maintenance. .

       In the cleaning process of single cast aluminum parts, due to cost control by parts suppliers and lack of anti-rust technology, the cleaning agent selected is not suitable for cleaning the surface of cast aluminum parts, has no anti-rust function, and will accelerate the corrosion of aluminum cast parts. Process parameters such as cleaning agent concentration and water change cycles are not strictly controlled, resulting in incomplete cleaning of the release agent and dust on the surface of the aluminum castings, increasing the risk of corrosion.

       In the rinsing process of single cast aluminum parts, no anti-rust agent was added and the parts were not dried, leaving cleaning agent, water, and dust remaining on the surface of the aluminum parts to form corrosion cores, accelerating the corrosion of the aluminum parts.

       Cast aluminum single parts and assemblies are not packaged with anti-rust materials before storage and transportation, resulting in out-of-control rust prevention during storage and transportation. Corrosion is more likely to occur during the rainy season or under shipping conditions. Considering the transportation cost, the mode of transportation is mostly sea transportation. The anti-rust period of sea transportation takes longer. Sea transportation has the characteristics of high concentration of corrosive media. The air contains sodium chloride, potassium chloride and other substances. In addition, the humidity at sea is high. Cast aluminum parts are prone to corrosion, and as the exposure time increases, the corrosion phenomenon gradually increases.

       Before the vehicle is sold, some vehicles may be parked in the open air for a long time or transported over long distances. Without anti-rust measures, the cast aluminum parts are prone to corrosion. Taking aluminum parts in the engine area as an example, cast aluminum engine cylinder head covers, chain room covers, suspension and other assemblies are more likely to corrode in a relatively closed environment. The temperature and humidity curves in the engine compartment of automobiles recorded by temperature and humidity recorders during the rainy season in northern China are shown in Figures 2 and 3.

 

       As can be seen from Figure 3, the maximum relative humidity range of a vehicle when parked is 70% - 100%, and the maximum relative humidity range when driving is 60% - 70%. The risk of corrosion of vehicles when parked in the open air for a long time is higher. Theoretical analysis and practical verification of corrosion conditions The situation is exactly the same.

       After the vehicle is sold, the temperature in the engine compartment will exceed 90°C during driving. At the same time, accompanied by rapid air flow, the relative humidity drops sharply. In Figure 3, after the vehicle is sold, the relative humidity in the engine compartment during normal use is at least 5%. , the risk of corrosion of aluminum parts is extremely low, and the lowest relative humidity in the engine compartment of a vehicle parked in the open is about 35%. From the above analysis, it can be seen that the real causes of corrosion of aluminum castings are complex and occur in multiple links. It is necessary to formulate a complete anti-rust process and select appropriate anti-rust materials to effectively prevent corrosion of aluminum castings.

4. Anti-rust solution

       After systematic corrosion cause analysis, anti-rust plans were formulated for each risk link and verified in the laboratory. Anti-rust technology was integrated into all aspects of aluminum casting manufacturing, storage, transportation, and assembly.

4.1 Anti-rust materials

       Anti-rust materials are divided into ferrous metal anti-rust materials and non-ferrous metal anti-rust materials according to the type of metal they protect. Anti-rust materials on the market often use water, oil, powder, paper, and plastic film as carriers, and add a certain amount of corrosion inhibitors to achieve anti-rust effects. Different types and dosages of corrosion inhibitors have different anti-rust effects. Through a series of tests during the approval process of anti-rust materials and compatibility tests between materials, 3 anti-rust materials suitable for die-cast aluminum alloys were screened out from more than 20 types of anti-rust materials in 3 categories (Table 1). Including cleaning agents, water-based anti-rust agents, vapor-phase anti-rust films, etc. Its main testing items include rust prevention, corrosiveness, vapor-phase anti-rust screening tests, etc.

       The cast aluminum test piece after the series of anti-rust tests is shown in Figure 4. The sample in Figure 4a shows a large number of white corrosion products, the sample in Figure 4b shows a large number of black spot corrosion products, and the sample in Figure 4c is intact. The actual vehicle verification is shown in Figure 5. In Figure 5a, a large amount of white corrosion products appear on the aluminum casting, and in Figure 5b, the aluminum casting is intact. Choosing appropriate anti-rust materials will ensure that aluminum castings will not corrode.

       By adding cleaning agents, water-based anti-rust agents and vapor-phase anti-rust materials in the single-piece manufacturing stage, adding vapor-phase anti-rust packaging in assembly storage and transportation, and spraying water-based anti-rust agents in the vehicle assembly, storage and transportation process, the die-casting process is eliminated. Corrosion problems of aluminum alloy parts.

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4.2 Anti-rust process

       Compared with the process in Figure 1, the process shown in Figure 6 fully considers the anti-rust technology of aluminum castings. Corresponding anti-rust measures have been added during the manufacturing of individual parts, assembly of the assembly, vehicle assembly, storage and transportation.

4.2.1 Single piece manufacturing

       Adding 3% weak alkaline cleaning agent in the cleaning process (see Table 2 for cleaning agent performance) can effectively remove the release agent remaining in the casting process and prevent corrosion problems caused by the release agent remaining on the surface of die-cast aluminum alloy parts.

       As can be seen from Table 2, acidic cleaning agents have a certain corrosive effect on the surface of die-cast aluminum alloy parts and slightly affect the appearance of the cast aluminum parts. The weakly alkaline cleaning agent has excellent performance and meets the technical specifications. In the rinsing process, 3% water-based anti-rust agent is added (see Table 3 for the performance of water-based anti-rust agent), that is, the cast aluminum parts are rust-proofed at the same time as rinsing, so that the aluminum parts can be made rust-proof without increasing the length of the process. An anti-rust film is formed on the surface, which does not affect the size and appearance of the aluminum parts and prevents corrosion problems before packaging.

Picture As can be seen from Table 3, the water-based anti-rust agent A meets the technical specifications and can ensure that the cast aluminum parts will not suffer from corrosion problems before being packaged for anti-rust. After the rinsing and rust prevention is completed, a drying process is added. The purpose is to effectively remove the moisture on the surface of the aluminum parts, that is, in the gap grooves. First, automatic line baking is used, and then the grooves and gaps are purged with clean compressed air. After the aluminum castings are completely dried, the relative humidity inside the rust-proof packaging is reduced from 80% to 30% at room temperature, effectively reducing the risk of corrosion due to high relative humidity inside the packaging.

       Single-piece packaging eliminates ordinary plastic films and uses vapor-phase anti-rust packaging materials. The vapor-phase anti-rust packaging materials continuously emit vapor corrosion inhibitor (Volatile Corrosion Inhibitor, VCI) into the interior of the package, effectively preventing corrosion during parts storage and transportation (Table 4 ), and has a better anti-rust effect on parts with complex structures, and the anti-rust period can generally exceed 3 months.

       It can be seen from Table 4 that after the aluminum castings are packaged with vapor-phase anti-rust film, there is no corrosion problem on the surface after being left in the natural state for 3 months. When ordinary plastic films are used, obvious corrosion problems occur.

4.2.2 Assembly assembly

       The same as the single-piece vapor phase rust prevention packaging, the assembled aluminum parts assembly is packaged in vapor phase rust prevention. For larger assemblies (such as engine assemblies), a certain amount of desiccant needs to be added inside the package to ensure that the relative humidity is maintained at a low level. Make the vapor phase anti-rust bag into the shape of a shower cap that can be closed on one side (Figure 7), and carry out local anti-rust packaging for areas involving aluminum castings. Local vapor-phase anti-rust has three advantages: First, local anti-rust is more targeted , Reduce the consumption of anti-rust materials; second, the packaging volume is small and easy to operate; third, the anti-rust bag with a closed mouth can effectively prevent the escape of vapor phase corrosion inhibitor and ensure a sufficient anti-rust period. Comparative test results prove that local anti-rust measures are effective.

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4.2.3 Vehicle assembly, storage and transportation

       In order to prevent the entire vehicle from being stored in the open air for a long time after assembly, it is necessary to spray water-based rust inhibitor on the relevant cast aluminum parts before the entire vehicle is assembled off the assembly line, after entering the open-air parking lot, and before arriving at the 4S store for sale, to ensure that the entire vehicle Aluminum castings are free from corrosion before sale. After the anti-rust measures were implemented, by tracking the storage and transportation of 1,000 complete vehicles in China, no corrosion problems occurred in die-cast aluminum alloy parts, and the measures were verified to be effective. After the vehicle is delivered to the user, due to frequent starting and driving, the temperature and humidity in the engine compartment will not be continuously high, and there will be no corrosion problem of die-cast aluminum alloy parts.

4.2.4 Others

       According to the corrosion mechanism of aluminum castings, it is prohibited to touch aluminum parts with bare hands in all aspects of manufacturing, storage, transportation, and assembly of aluminum castings. Avoid sweaty hands (containing lactic acid) from contacting aluminum parts; avoid rain and dust, and carry out preventive measures as early as possible. Rust packaging and other processing.

5. Conclusion

       This article conducts in-depth research on the corrosion mechanism, corrosion causes, anti-rust materials and processes of die-cast aluminum alloy parts, and adds corresponding anti-rust measures to each relevant process to achieve the purpose of anti-rust. However, the anti-rust operation is complicated and is not conducive to production control. In the future, we should start from the die-cast aluminum alloy material itself and develop die-cast aluminum alloy materials with higher corrosion resistance to eliminate the corrosion problem of die-cast aluminum alloy parts from the root cause.