Just like atmospheric environment, which is different for corrosivity depending on where people are located in the world, the same is for metal corrosion in automotive parts: for example in Sweden the automotive environment is quite aggressive because of humid climate and of the use of de-icing salts causing metals corrosion.
Corrosion is a natural process, that changes metals to a more stable form, it is the gradual materials destruction due to chemical or electrochemical reaction with the environment.
Metal corrosion in automotive parts: how is it created?
What is corrosion in details?
Most metals are unstable in natural or industrial environments; for this reason they return to their previous form: this process is called corrosion.
Electrochemical corrosion variables are linked to a conductive material which is the metal, an ion conductive media that usually is water, an anodic and cathodic reaction. Metal corrosion is driven by coupled anodic and cathodic reactions which occur during the interface between metal and aqueous environment.
There are different types of corrosion: hereunder we are going to explain which are and analyze deeply metal corrosion in automotive parts.
First of all we will explain different corrosion types, then we will focus on the differences between every environment and which will be the approximate corrosion rate.
Corrosion types
Different types of corrosion can occur depending on metal nature that is involved and the surrounding environment.
The types of corrosion:
- Uniform corrosion
- Galvanic corrosion
- Pitting corrosion
- Crevice corrosion
- Deposit corrosion
Uniform corrosion
This is the most important kind of corrosion: its characteristic is corrosion rate approximately over the entire exposed surface.
Galvanic corrosion
Galvanic corrosion takes place when two different metals come in contact with an electrolytic presence on the surface. The more noble metal acts as cathode and the less noble as anode. The more noble metal is protected against corrosion, while the less noble is not.
Pitting corrosion
This type of corrosion occurs on passivated metals that if exposed to aqueous solutions containing aggressive additives. The passive layer is destroyed by the aggressive additive and when the metal dissolves it creates a cavity, which is the anodic area, while the metal passivated area acts as cathode.
Crevice corrosion
When a small crevice is created on a surface, the metal inside of it is subjected to a more aggressive environment due to oxygen depletion, acidification or increasing concentration of CI (chloride ions) ions. If there is not oxygen available for surface re-passivation, the passive layer can be destroyed and the metal corrodes in the crevice.
Deposit corrosion
The cause of deposit corrosion is water retention under the deposit. Due to this situation is common to have a corrosive environment such as under road mud for exposed car parts.
Corrosion environments
There are different kind of environments related to different corrosion types such as rural, marine, industrial, urban and automotive. In particular, we are going to deeply analyze the correlation between metal corrosion in automotive parts and environments.
Rural Atmosphere
Low amounts of pollutants with a deposition rate of SOA<10 mg/m2 and NaCI<5 mg/ m2. Approximate corrosion rate: 0.2 – 2 µm/year.
Marine Atmosphere
High amount of chlorides with an average deposition rate per year of NaCI between 5 and 500 mg/ m2 each day and up to 1500 mg/ m2 a day if exposed to a lot of sea spray. Approximate corrosion rate: 0.5/8 µm/year.
Industrial Atmosphere
Can contain many different kinds of pollutants and has a high deposition rate of SO2, up to 200 mg/ m2 a day. Approximate corrosion rate: 1-16 µm/year.
Urban Atmosphere
High amounts of SO2 and soot with a deposition rate of SO2B of 10-80 mg/ m2 day. Approximate corrosion rate: 1-16 µm/year.
Automotive environment
Automotive environment is quite aggressive and causes metal corrosion in automotive parts due to high pollution levels and due to de-icing salts. SAE, The Society of Automotive Engineers, estimated coating corrosion rate to be about 8.5 µm/year. This is the same range as in marine environments.
Road mud
There is a lot of road mud which can be attached to an exposed metal such as automotive parts and create corrosion. When this mud or dust is deposited on component surface creates a longer TOWC (Time of Wetness). The area under this deposit can act as anode and the un-deposited as cathode.
De-icing salts
These particular kind of salts are used in order to lower snow melting temperature and ice on roads to increase traction between tires and road.
One of the most commonly used de-icing salt is sodium chloride (NaCI), in fact this particular solution has a strong corrosive effect.
Summarizing, during die casting, is important to deeply analyze uses and applications for products in order to adjust their functional requirements before production to increase their corrosion resistance and avoid any possible issue for functional or aesthetical aspects.
As a result, one of the most common way to protect vehicles from corrosion damage is to apply protective coatings on surface. In fact die casting surface finishing are industrial processes able to modify manufactured items surface in order to improve their functional and aesthetic characteristics. Indeed with surface finishing the die caster can solve and prevent any defects caused by external agents such as, for example, corrosion.
Moreover in order to obtain a product aligned to clients requirements is necessary to have a perfect cast product, with a correct manufacturing conformation, and a surface without any defect due to die casting phase.
To obtain all these specifications the collaboration between designer and clients is crucial: this collaboration generates co-design service. In fact with co-design, and its first phase called simulation analysis, designers can suggest clients the best way to proceed in order to avoid any possible future issues.
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Notes:
A SO: Sulfure oxide
B TOW: is an estimated parameter based on the length of time when the relative humidity is greater than 80% at a temperature greater than 0 0C. TOW is used by many researchers as a parameter to correlate corrosion rates with atmospheric factors.
C SO2: Sulfure dioxide
