Thought corrosion was only about those unsightly patches of red oxide that plague steel and iron products in coastal areas and offshore applications? Think again. 

Corrosion is present everywhere, and can occur in even the most bone dry, landlocked countries. Understandably, coastal and offshore applications are the ones most at risk, due to their exposure to salt water. However, corrosion isn’t limited to salt water exposure. Cleaning agents, high humidity and ‘dirty’ environments, like sewage and mining, all aggravate the corrosion process.

Corrosion can’t be avoided, only mitigated. The best form of ‘protection’ is to already factor in the impact of corrosion from the start of designing a product, ensuring the materials used for an application resist corrosion as much as possible. And more importantly, to make sure that the metals used don’t accelerate the corrosion process by reacting against each other – a phenomenon also known as galvanic corrosion. 

Protecting products from corrosion 

As a rule of thumb materials with a big difference in electrode potentials should not be combined. If, for example, copper and stainless steel alloys are coupled, a protective coating is needed to reduce corrosion. Aluminium alloys and copper shouldn’t be combined, especially where there is a higher pH due to the environment’s salinity. 

Unawareness of these galvanic reactions can have devastating financial and safety effects, tarnishing a company’s image. A US oil refinery, for example, once suffered major failures from caustic cracking caused by corrosion, resulting in costs of around US$500 million. 

Even though bolts are components in the construction process, they must also be designed with care. If they corrode, the larger structure or product will also fall apart. Therefore, fasteners must be dependable. Corroded fasteners not only result in metal loss and possible failure, but in case of high strength fasteners, cracking and sudden failure. Sometimes it is not practical to use corrosion resistant fasteners, or to use the same material of all parts in the joint, then other methods of corrosion mitigation, like coatings, cathodic protection or inhibitors must be used to protect them.

Coatings and other alternatives 

Nord-Lock Group offers a wide range of products and services to provide safe bolted joints. It is aware of the problems that galvanic corrosion can create and often supplies customised products. According to Harlen Seow, technical manager at Nord-Lock APAC, choosing the right material and corrosion protection for the bolt is crucial: “Customers often don’t understand why we ask them so many questions about the materials they are using, and the environment their product will be used in. But we have to know all these details to supply the right products.”

Zinc flake coatings  are currently the most popular protection for steel bolts and washers. These coatings are applied like paint, and then baked to create a barrier; if more layers are added, they also act as a friction-proof coating. Other options include Teflon coatings and hot dip galvanising. 

“There are a few simple rules to follow if you would like to mitigate corrosion in a bolted connection. But please remember, corrosion cannot be avoided only mitigated so do not forget to make regular inspections."

Source: Fastener and Fixing Technology
Date: 07.02.18