Oxidation is a natural phenomenon that occurs when there is contact between an oxygen-rich atmosphere and a metallic material. Corrosion is the effect of chemical and electrochemical phenomena which cause the destruction or deterioration of the metallic material.
Corrosion protection becomes one of the most important aspects to consider, especially in a fastening system, when designing a joint in particularly aggressive environments.
Until a few years ago, hexavalent chromium was one of the basic components of electrolytic zinc coatings. Numerous directives - RoHS; WEEE and ELV - have banned the presence of heavy metals such as lead, mercury, cadmium, PBB or PBDE and hexavalent chromium
Since 2006, none of FAR products have contained heavy metals.
Each type of product has a specific zinc coating, which is conceived for its most common applications and indicated in the technical datasheet of the individual fastener by means of the symbols adopted by the leading European auto makers.
Zinc coatings other than the ones provided can be used, subject to a technical feasibility assessment. The resistance of a zinc-based surface coating can be increased using a sealant.
There are also other possible anticorrosive treatments for rivets and inserts, such as:
- Nickel plating
- Zinc-Nickel plating
Corrosion resistance is evaluated with a salt fog test. This is an accelerated corrosion test for qualitatively assessing the corrosion resistance of the protection provided by the coating.
There is no way of directly comparing the results of this test and the actual behaviour of the product once applied due to the numerous factors that influence corrosion, for example: whether the environment is highly oxidizing or not, the nature of the materials in contact, component deformation, which may partly ruin the surface coating layer, etc.
For such reasons it is not possible to guarantee any specific corrosion resistance beforehand.
However, for a wholly approximate indication, reference can be made to the following table:
|Coating type||Electrolytic zinc plating||Number of hours in the salt fog chamber
(naked-eye assessment of outer surface)
|White rust on zinc||Red rust on zinc|
|FeZn 5 II||Bright light-coloured passivation||24||72|
|FeZn7II||Bright light-coloured passivation||24||96|
|FeZn7IV S||Iridescent passivation with sealer||120||360|
Finally, it is advisable not to use components that have been zinc plated at temperatures above 250°C, as they are subject to degradation, albeit partial, of the zinc coating layer.