“Ask the Experts” is our way of answering some of the common questions we receive from customers in day-to-day business. We rely on a broad network of marine piping industry experts to get to the bottom of some of the challenging questions in ship repair. Have a question you want answered? Email it to [email protected].
What is electrolysis, and is it making my copper-nickel corrode faster than normal?
Electrolysis happens when a DC current is applied to two types of metal and an electrolyte. What typically happens in a marine vessel is actually something called galvanic corrosion, also known as dissimilar metal corrosion. This is a problem that is very common in the marine pipe, valve, and fitting industry.
It’s not electrolysis? What is Galvanic corrosion?
Galvanic corrosion is when two metals of different electric conductivity are connected and then exposed to an electrolyte such as sea water. The more active material (has lower electrical resistance) is called the anodic metal, and will corrode faster than it normally would. The metal that is less active (has higher electrical resistance) is known as the cathodic metal, and will corrode at a slower-than-standard rate. Galvanic corrosion can also occur when certain non-metal materials, such as carbon or cellulosic reinforced plastics, are connected to a metal.
Simplified: You have stainless steel connected to copper-nickel. Sea water runs through it. The movement of the sea water creates a small electrical charge, and copper-nickel conducts electricity better than stainless steel. The copper-nickel absorbs the charge and corrodes faster than it normally would. Since the copper-nickel is absorbing the charge, the more noble metal, stainless steel, corrodes slower than normal.
I don’t want my parts to corrode! What do I do?
There are some measures you can take to avoid having problems with galvanic corrosion. If you must mix metals, use a design in which the cathode is much smaller than the anode. For example, use stainless steel bolts to fasten aluminum but not the other way around. Another way to avoid galvanic corrosion is to use a gasket or washer to separate the two pieces of metal that is compatible with both pieces. This will alleviate some of the electrochemical differences and slow corrosion.
You can also use a sacrificial element. This involves using a plating in the anodic side that is smaller and more active than the anodic metal. The sacrificial element will take the punishment so that the anodic metal will last longer. In some cases, you can use an organic coating to protect the inside of the anodic metal. However, not all conditions allow for this.
I’m planning out a new project. Which metals don’t mix?
For harsh conditions in which the metal is exposed to humidity and salty environments, there should not be more than a .15 V difference in the anodic index. This includes most maritime applications. Lower indexes can be used in vessels on fresh water or in storage and controlled environments. When you need to replace those corroded parts, connect with Tork Systems, Inc. We have a huge selection of the best copper-nickel pipes, fittings, and flanges in the country!