Corrosion Resistance

Corrosion resistance can be affected by the base material, adjacent materials and by the environmental conditions surrounding the material.

Base materials like 300-series stainless steel have a very high corrosion resistance; however, if you place them with a zinc-plated or galvanized metal, there will be a large increase in the corrosion of the metals due to the galvanic corrosion.

Designs sometimes cause dissimilar metals to be in contact within an assembly.  Increased variance of the two metals’ anodic index cause accelerated corrosion of the more anodic metal.  The variance in anodic index can be controlled somewhat through coatings (paint, epoxy, powder coat, etc) and plating (zinc, chrome, etc).

The table below provides a generalize guide for predicting the anodic corrosion two dissimilar metals are prone to.  The service environment also plays a role:

• For harsh environments: outdoors, high humidity, and salt environments fall into this category. Typically there should be not more than 0.15 V difference in the “Anodic Index”. For example; gold – silver would have a difference of 0.15V being acceptable.
• For normal environments: storage in warehouses or non-temperature and humidity controlled environments. Typically there should not be more than 0.25 V difference in the “Anodic Index”.
• For controlled environments: where the temperature and humidity controlled, 0.50 V can be tolerated. Caution should be maintained when deciding for this application as humidity and temperature do vary from regions.

Likewise, if you place the material on a bridge over the sea, the salt spray will vastly decrease the corrosion resistance of the material.