The standard "VW PV 1015 Bimetallic corrosion of elastomers and plastics on light metals - Specific volume resistance test" developed by the Volkswagen automobile company describes the requirements for elastomer parts and plastics in body white or body components that come into contact with light metals (preferably aluminum and magnesium) and for window glass adhesives. This standard also specifies the requirements for elastomer parts and plastics and window glass adhesives that come into contact with light metals (wall thickness up to 1,2 mm) on the outside of the body. This standard is valid for sample and production monitoring tests.
Light metals, such as aluminium or magnesium, are likely to corrode in the presence of aqueous electrolytes if they come into contact with more noble metals or n-type solids, such as elastomers or plastics. Elastomers and plastics must have a satisfactory specific volume resistivity to prevent such corrosion. This also applies to materials that come into contact with painted light metal surfaces, since paint layers do not provide complete protection against bimetallic corrosion due to possible damage or defects in the paint.
Sample test plates for the tests shall be 120 mm x 120 mm x 2 mm in size. The surfaces of the test plates shall be cleaned with ethanol before aging in a standard atmosphere. The specific volume resistivity shall be determined in a standard atmosphere according to the standard “IEC 60093 Test methods for volume resistivity and surface resistivity of solid electrical insulating materials”. According to the standard “ISO 554 Standard atmospheres for conditioning and/or testing - Specification” (23 degrees Celsius and 50 percent relative humidity).
For elastomers and plastics, the specific volume resistivity requirement after 48 hours of aging and contact in a standard atmosphere shall be at least 104 Ω cm for aluminium and 106 Ω cm for magnesium. For window glass adhesives, the specific volume resistivity requirement after 48 hours of aging in a standard atmosphere shall be at least 106 Ω cm (regardless of the type of light metal).
Elastomer parts and glass adhesives must be bonded to pre-treated and painted sheet metal under production conditions. The test period is 1210 cycles according to the standard “VW PV 60 Body and attachment parts / suspension parts - Corrosion test”. After the test, there should be no visible damage to the sheet metal caused by bimetallic corrosion.
Bimetallic corrosion is also called galvanic corrosion and is an electrochemical process in which a metal corrodes primarily when in electrical contact with another metal in the presence of an electrolyte. In other words, bimetallic corrosion occurs when two different metals come into contact with each other in the presence of an electrolyte such as water. The interaction between the two metals, known as electrolysis, causes corrosion.
In short, bimetallic corrosion is caused by an electrochemical process that occurs when two metals come into contact with each other in the presence of an electrolyte such as rain, soil or seawater, causing an electrical reaction. This electrical reaction causes one metal to become the anode (negatively charged) while the other, more noble (less corrosive) metal becomes the cathode (positively charged). An electric current flows from the anode to the cathode, causing the anode to corrode at an accelerated rate. Corrosion weakens the chemical structure of the metal and can cause structural elements to deteriorate in just a few years. This can therefore weaken the entire structure, jeopardizing human safety and causing disasters. In order to prevent bimetallic corrosion and minimize the risk of bimetallic corrosion of fasteners, the fastener must be matched to the metal to which it will fix the surface metal.
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