Corrosion is the process through which metal degrades in the presence of various oxidizing agents in the surrounding environment. Corrosion takes many forms and can have many root causes.
One common example of this is rust, during which iron oxides form in the presence of moisture. Corrosion can cause serious headaches for the manufacturers of buildings, boats, aircrafts, cars, and most other metal products. For instance, when metal is used as part of a bridge, the structural integrity of that metal, which can be compromised by corrosion, is crucial to the safety of the people using the bridge.
Preventing corrosion isn’t always an easy task, so we’ve devise three parts to help you fight rust and maintain healthy metal products. This is the first part of the series, which focuses on learning how to protect metals from the threat of corrosion and how to slow it down if it does start.
Understanding and preventing Common Types of Metal Corrosion
– How to prevent uniform attack corrosion by protecting the metal
Uniform attack corrosion (sometimes shortened to “uniform” corrosion) is a type of corrosion that occurs, appropriately, in a uniform fashion over an exposed metal surface. In this case, the entire surface of the metal is under attack from corrosion so it develops at a uniform rate.
For example, if an unprotected iron roof is regularly exposed to rain, the entire roof surface will come into contact with roughly the same amount of water and thus will corrode at a uniform rate. The easiest way to protect against uniform attack corrosion is usually to put a protective barrier between the metal and the corroding agents. This can be a wide variety of things – paint, oil sealants or an electrochemical solution like a galvanized zinc coating all work.
If your metal is underground or regularly immersed in water, consider cathodic protection to prevent corrosion.
– How to prevent galvanic corrosion by halting ion flow from one metal to another
Galvanic corrosion can occur regardless of the physical strength of the metals involved. It occurs when two metals with differing electrode potentials are in contact with one another in the presence of an electrolyte (like saltwater) that creates an electrical conducting path between the two. When this happens, metal ions flow from the more-active metal to the less-active metal, causing the more-active metal to corrode at an accelerated rate and the less-active metal to corrode at a slower rate. In practical terms, this means that corrosion will develop on the more-active metal at the point of contact between the two metals.
Any method of protection that prevents ion flow between two metals can potentially halt galvanic corrosion. We recommend giving the metals a protective coating to stop electrolytes creating a conducting path. Eletrochemical protection processes like galvanisation and anodising also work well. It’s also possible to stop galvanic corrosion by electrically insulating the area of the metals that are in contact with one another.
Additionally, the use of cathodic protection or a sacrificial anode can protect important metals from galvanic corrosion. We go into more detail on this below.
– How to prevent pitting corrosion by protecting the metal surface, avoiding environmental chloride sources and avoiding nicks and scratches
Pitting is a form of corrosion that takes place at the microscopic scale, but can have large-scale consequences. It’s concerning particularly for metals that derive their corrosion resistance from a thin layer of passive compounds on their surface, as this form of corrosion can lead to structural failures in situations where the protective layer would normally prevent them.
Pitting occurs when a small part of the metal loses its protective passive layer. When this happens, galvanic corrosion occurs at a microscopic scale, leading to the formation of tiny holes on the metal. Within this hole, the local environment becomes highly acidic and this accelerates the process. Pitting is usually prevented by applying a protective coating on the metal surface and/or using cathodic protection.
Exposure to an environment high in chlorides (like, for example, salt water) is known to accelerate the pitting process.
– How to prevent stress corrosion cracking by using only safe loads and/or annealing
Stress corrosion cracking (SCC) is a rare form of corrosion-related structural failure that mostly concerns engineers who build structures intended to support big loads.
In the event of SCC, a load bearing metal forms cracks and fractures below its specified load limit and in severe cases, at a fraction of the limit. When corrosive ions and microscopic cracks appear, the heavy loads cause tensile stress which causes the crack to grow and potentially cause eventual structural failure.
SCC is particularly dangerous because it can occur even in the presence of substances that are naturally only very mildly corrosive to the metal. This means the dangerous corrosion occurs even while the rest of the metal’s surface appears unaffected.
Preventing SCC is partly a design issue. For instance, by choosing a material that is SCC-resistant in the environment in which the metal will operate and ensuring that the metal material is properly stress-tested can help prevent SCC. Additionally, the process of annealing a metal can eliminate residual stresses from its manufacture.
SCC is known to be exacerbated by high temperatures and the presence of liquid containing dissolved chlorides.