Rosemont, IL — When specifying electrical products for harsh environments, choosing the right materials to ensure adequate corrosion resistance is crucial. In offshore oil and gas operations, equipment is under constant exposure to seawater and salt spray, which are highly corrosive due to the autocatalytic action of sodium chloride and other dissolved chlorides. Corrosive substances such as hydrogen sulphide and carbon dioxide also occur naturally in oil and gas fields. Other corrosives that affect a wide range of industries include chlorine, bromine, hydrochloric acid and ammonia.
Corrosion can take many forms, and correct selection of materials and finishes is key to preventing each type. Common corrosion control methods include:
Selecting materials according to their galvanic properties. Different metals and alloys have different electrode potentials. When two different metals are electrically connected in the presence of an electrolyte, such as seawater, the more active metal will become anodic — losing electrons and increasing its oxidation state in a process known as galvanic corrosion.
The galvanic series ranks metals from noble to active, based on the strength of ion bonding at the surface. A relatively more noble metal, such as stainless steel, will resist corrosion better than a more active metal, such as cast iron. Galvanic corrosion can be minimized by selecting metals close together in the galvanic series. Alternatively, a more active metal can be used as a sacrificial anode that attracts corrosion in order to protect the more noble metal from attack.
Using protective coatings. Various finishes can be applied to help isolate metallic surfaces from the surrounding corrosive environment. The most familiar example is ordinary paint applied to steel to prevent rust, but there are many other methods, including baked enamel, epoxy powder coat and PVC coating.
For effective protection, coatings must be applied properly and protected against damage during installation and use. The zinc surface on galvanized steel serves both as a protective coating and, if damaged, a sacrificial anode that will corrode in preference to the exposed steel.
Taking advantage of passivation. Certain metals form a layer of metal oxide on the surface, a few molecules thick, in a process known as passivation. This occurs naturally, but the process can also be enhanced through chemical passivation treatments or anodization. Unlike ordinary rust, the passivated layer is tightly bound to the surface, preventing any further penetration of oxygen or corrosive chemicals. The most familiar examples are aluminum and stainless steel, which form passivated layers of aluminum oxide and chrominum oxide, respectively.
If the surface is damaged, the passivated layer normally re-forms quickly. However, adverse conditions can defeat the process. For example, pitting corrosion can occur in aluminum exposed to seawater when chloride ions interfere with passivation. Welded stainless steel can corrode when the carbon content is sufficiently high to form chromium carbides, depleting the chromium available for passivation in the weld zone and enabling a galvanic reaction between areas with differing chromium content. When properly selected for the intended application, however, passivated materials can provide excellent protection even in highly corrosive environments.
Choosing nonmetallic components. Manufacturers are increasingly offering nonmetallic products that are completely impervious to the environments and substances that can corrode metals. Nonmetallic materials may be attacked by specific chemicals, however, depending on the composition of the plastics used. The vast majority of applications will never be exposed to these substances. Prolonged exposure to UV radiation can also degrade plastics, and users in high-UV locations should account for this in their materials specifications.
Not all industries have the same corrosion problems, but all industries do need to take the possibility of corrosion seriously and choose the right materials and products for their particular operations and environment. The NACE International, The Corrosion Society estimates the worldwide annual cost of corrosion at $2.2 trillion, more than 3% of global GDP. Any money that can be saved by preventing corrosion — including the cost of downtime, labour and new equipment to replace failed parts — directly improves profitability.
To learn more, visit www.appletonelec.com. Appleton Group is a division of Emerson Industrial Automation and is headquartered in Rosemont, IL.