In the process of die casting, two characteristics of the die cast alloy need special design considerations. These characteristics are the galvanic potential and the response to long term loading. Electromotive series show that the die cast alloys are anodic, and that the chances of galvanic deterioration is very high when they come in contact with alloys that contain copper or iron.
Galvanic Potential
When a die casting part come in direct contact with dissimilar metals, there are chances of galvanic corrosion. This chance of galvanic corrosion can be controlled by using proper design measures. Studies prove that all the combinations of dissimilar metals are prone to galvanic corrosion. The severity and tendency of the issue depends on the relative position of the metal in the electromotive series. Die casters usually make use of two principles to determine the potential galvanic issues in die casting:
- Anodic metals are usually affected by galvanic corrosion.
- The wider the spread of the corrosion, greater will be the galvanic potential.
Rate of Galvanic Corrosion
Usually, lab tests are conducted to find the galvanic corrosion rate for a combination of metals. In these tests, the alloys are exposed to severe conditions like salt spray to determine the rate of galvanic corrosion.
You need to understand that the actual service conditions are not as severe as the conditions used in laboratory tests, so in many cases where the lab tests showed galvanic potential, no special measures are needed to avoid corrosion. For instance, salt tests conducted inside laboratories show that presence of copper in aluminum alloy 380 would make it incompatible to be used with magnesium alloy theoretically. However, in reality, there is no need to use a galvanic insulation between 380 aluminum parts and AZ91D magnesium alloy parts. It is the duty of the die caster to help clients find the extent of galvanic protection needed for the particular application.
Four conditions should coexist for the galvanic corrosion to happen. These are:
- The presence of an anode or a corroding material;
- Presence of a cathode of dissimilar metal;
- Electrical contact between the metals, typically, touching each other; and
- Presence of al electrolyte or a liquid path between the metals.
As all the above conditions should coexist, eliminating any one of the above will help avoid the issue of galvanic corrosion of die cast parts. Die casters use any one of the below steps to prevent galvanic corrosion.
- Minimize or avoid electrolyte accumulation.
- Selecting metals that are much compatible.
- Insulate metals against contact.