Avoiding galvanic corrosion
Too little focus on galvanic corrosion can cause a design to become very poor, and this is an outcome that costs lots of money to repair. But galvanic corrosion can be avoided rather easily.
Galvanic corrosion, also known as dissimilar-metal corrosion, is one of the three most common types of corrosion in aluminium. The others are pitting corrosion and crevice corrosion. My focus in this article is galvanic corrosion.
Let me step back and point out that aluminium has great corrosion protection. This is fact. Another fact is that galvanic corrosion in aluminium does not occur indoors and in other dry or inland atmospheres.
That said, galvanic corrosion of aluminium occurs:
- Where there is contact with a more noble metal, such as copper and carbon steel. After magnesium and zinc, aluminium is normally the least noble metal, in combinations with other metals.
- Where, at the same time, there is an electrolyte (with good conductivity) between the metals and the supply of oxygen.
In other words, the risk of galvanic corrosion in aluminium must always be considered in environments with high chloride levels, such as areas bordering the sea. Where aluminium is in contact with a more noble metal, and there is water, then there will be corrosion.
Electrical insulation and cathodic protection
I started off by saying that too little focus on galvanic corrosion can cause a design to become very poor, and showed how galvanic corrosion can occur.
Here are three ways you can prevent galvanic corrosion through improved design:
- Electrical insulation. You electrically insulate the metals from each other. The insulation needs to break all contact between the metals.
- Breaking the electrolytic bridge. Where insulation is difficult, in large constructions for example, an alternative solution is to keep an electrolytic bridge from forming between the metals. Painting can help you with this. I suggest coating the cathode surface, i.e. the most noble metal. You can also use an insulating layer between the metals.
- Cathodic protection. The most common way to do this is to mount an anode of a less noble material (often zinc) in direct metallic contact with the aluminium object to be protected. By doing this, you sacrifice the less noble material – it corrodes – for the aluminium. We call it a sacrificial anode. Another way to obtain cathodic protection is to connect the aluminium object to the negative pole of an exterior DC voltage source.
Replacing electroplated steel with hot dip galvanized steel
Some smart thinking will also allow you to avoid the risk of corrosion in the metallic combination aluminium and galvanized steel. And there is a risk, especially in aggressive environments.
At first, the zinc coating of the galvanized steel will prevent the aluminium from being attacked. This is good. Unfortunately, this protection disappears when the steel surface is exposed after the consumption of the zinc.
You can improve the system against such a corrosion attack simply by replacing the electroplated material with hot dip galvanized steel, because the hot dip metal has a thicker coating of zinc and provides longer protection.
My solution? Use hot dip galvanized material in combination with aluminium.