With 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. Material of many parts of car has been replaced from cast iron to die-cast aluminum alloy, which reduces quality of 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 body are not protected by paint coatings. For example, some aluminum parts inside engine compartment and on chassis will corrode before vehicle is sold. Although it does not affect its mechanical properties, it will seriously affect appearance quality, leading to consumer complaints and even claims. At present, there is little systematic in-depth research on corrosion problem of aluminum castings, lack of suitable anti-rust technology has been plaguing OEMs and aluminum casting parts suppliers. Based on corrosion phenomenon, this article systematically analyzes causes of corrosion, 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, forms operating documents and corporate standards. After implementation of various anti-rust measures, complaints caused by corrosion of die-cast aluminum alloy parts were eliminated.

There are three main reasons for corrosion of aluminum castings:
a. After aluminum parts are shot blasted, oxide film formed on the surface is not dense and cannot protect base material;
b. Aluminum element and Al2O3 are both amphoteric substances, 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. Dust structure is loose and porous. Under high humidity in summer and condensation in winter, it is easy to absorb moisture, sulfide, chloride, etc. in air to form acidic substances, causing corrosion of aluminum parts.

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

 

As can be seen from Figure 1, no anti-rust treatment is performed on 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 cleaning process of single cast aluminum parts, due to cost control by parts suppliers and lack of anti-rust technology, cleaning agent selected is not suitable for cleaning the surface of cast aluminum parts, has no anti-rust function, and will accelerate 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 release agent and dust on the surface of aluminum castings, increasing risk of corrosion.
In rinsing process of single cast aluminum parts, no anti-rust agent was added and parts were not dried, leaving cleaning agent, water, and dust remaining on the surface of aluminum parts to form corrosion cores, accelerating corrosion of 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 rainy season or under shipping conditions. Considering transportation cost, mode of transportation is mostly sea transportation. Anti-rust period of sea transportation takes longer. Sea transportation is characterized by high concentrations of corrosive media. Air contains substances such as sodium chloride and potassium chloride. Coupled with high humidity at sea, aluminum castings are prone to corrosion. As exposure time increases, corrosion phenomenon gradually increases.
Before vehicle is sold, some vehicles may be parked in open air for a long time or transported over long distances. Without anti-rust measures, cast aluminum parts are prone to corrosion. Taking aluminum parts in 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. Temperature and humidity curves in engine compartment of automobiles recorded by temperature and humidity recorders during rainy season in northern China are shown in Figures 2 and 3.

 

 

As can be seen from Figure 3, maximum relative humidity range of a vehicle when parked is 70% - 100%, and maximum relative humidity range when driving is 60% - 70%. Risk of corrosion is higher when vehicles are parked in open air for a long time. Theoretical analysis of corrosion conditions is completely consistent with actual verification.
After vehicle is sold, temperature in engine compartment will exceed 90℃ during driving. At the same time, accompanied by rapid air flow, relative humidity drops sharply. In Figure 3, after vehicle is sold, relative humidity in engine compartment is at least 5% during normal use, and risk of corrosion of aluminum parts is extremely low. However, relative humidity in engine compartment of a vehicle parked in the open is at a minimum of about 35%. From above analysis, it can be seen that 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.

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

Anti-rust materials are divided into ferrous metal anti-rust materials and non-ferrous metal anti-rust materials according to 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 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.
Table 1 Test results of key technical indicators of anti-rust materials

Cast aluminum test piece after series of anti-rust tests is shown in Figure 4. Sample in Figure 4a shows a large number of white corrosion products, sample in Figure 4b shows a large number of black spot corrosion products, and sample in Figure 4c is intact. Actual vehicle verification is shown in Figure 5. In Figure 5a, a large amount of white corrosion products appear in aluminum casting, and in Figure 5b, 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 single-piece manufacturing stage, adding vapor-phase anti-rust packaging in assembly storage and transportation, and spraying water-based anti-rust agents during vehicle assembly, storage and transportation, eliminates corrosion problem of die-cast aluminum alloy parts.

 

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

 

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

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 appearance of cast aluminum parts. Weakly alkaline cleaning agent has excellent performance and meets technical specifications. In rinsing process, 3% water-based anti-rust agent is added (see Table 3 for performance of water-based anti-rust agent), that is, cast aluminum parts are rust-proofed while rinsing, so that an anti-rust film is formed on the surface of aluminum parts without increasing length of process, without affecting size and appearance of aluminum parts, and preventing corrosion problems before packaging.
Table 3 Performance test results of water-based rust inhibitors

As can be seen from Table 3, water-based anti-rust agent A meets technical specifications and can ensure that cast aluminum parts will not suffer from corrosion problems before being packaged for anti-rust. After rinsing and rust prevention is completed, a drying process is added. Purpose is to effectively remove moisture on the surface of aluminum parts, that is, in gap grooves. First, automatic line baking is used, then grooves and gaps are purged with clean compressed air. After aluminum castings are completely dried, relative humidity inside anti-rust packaging at room temperature is reduced from 80% to 30%, effectively reducing risk of corrosion due to high relative humidity inside packaging.
Single-piece packaging eliminates ordinary plastic films and uses vapor-phase anti-rust packaging materials. Vapor-phase anti-rust packaging materials continuously emit vapor corrosion inhibitor (Volatile Corrosion Inhibitor, VCI) into interior of package, effectively preventing corrosion during parts storage and transportation (Table 4 ), has a better anti-rust effect on parts with complex structures, and anti-rust period can generally exceed 3 months.
Table 4 Comparison of actual anti-rust performance of single package

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

Same as single-piece vapor phase rust prevention packaging, 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 package to ensure that relative humidity is maintained at a low level. Make vapor phase anti-rust bag into shape of a shower cap that can be closed on one side (Figure 7), 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 and reduces consumption of anti-rust materials; second, packaging volume is small and easy to operate; third, anti-rust bag with a closed mouth can effectively prevent escape of gas phase corrosion inhibitor and ensure a sufficient anti-rust period. Comparative test results prove that local anti-rust measures are effective.

 

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

According to 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 contact with aluminum parts with sweaty hands (containing lactic acid); avoid rain and dust, carry out anti-rust packaging and other treatments as soon as possible.

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