Corrosion of metals is an electrochemical reaction that involves changes in both the metal and the environment in contact with the metal. While the mechanisms of corrosion are the same on a microscopic level, various microstructure, composition, and mechanical design issues will lead to different manifestations of corrosion.
There are seven common types of metal corrosion
- Stress corrosion cracking (SCC)
Uniform corrosion occurs over the entire exposed surface of a metal. Rust on a steel structure or the green patina on a copper roof are examples of uniform corrosion. The driving force for this type of corrosion is the electrochemical activity of the metal in the environment to which the metal is exposed.
Galvanic corrosion occurs near the junction between two dissimilar metals.The driving force for the corrosion reaction is the difference in electrode potentials between the two metals.
Crevice corrosion occurs in crevices between components and also under polymer coatings and adhesives. The driving force for the corrosion is the difference between the oxygen concentration inside the crevice and outside the crevice.
Pitting occurs in metals that are normally passive, when the passive layer breaks down. Examples of passive metals are aluminum and stainless steel. Pitting is a problem if it leads to weakening or perforation of the metal. In applications where appearance is important pitting is a problem
Interganular corrosion involves corrosion along the grain boundaries of the affected metal. The result is that the metal grains fall away and the metal is weakened. Austenitic stainless steels and precipitation strengthened aluminum alloys such as 2xxx alloys are examples of metals that can suffer from intergranular corrosion if the alloys are not properly processed and if they are exposed to corrosive environments.
Stress corrosion cracking involves the combined action of stress and exposure to a corrosive environment. In most cases, the stress or environment by themselves are insufficient to cause degradation of the metal. That is, the stress is below the metal’s yield strength and the metal would not corrode in the specific environment if the stress was absent.
Dealloying is the selective leaching of one element from an alloy. This results in the formation of a porous structure that is not strong enough to support the applied mechanical loads. One common example is dezincification of brass alloys used for plumbing, where the zinc is leached out of the alloy.
The specific type of corrosion that occurs depends on the several factors including metal composition, metal microstructure, environment, component geometry, stress on the component, contact between metals, and the manner in which components are joined together.
In some cases, the root cause of metal corrosion failures is selection of materials that are inherently incompatible with the environment. In other cases, corrosion is a result of mechanical design, where incompatible metals are joined together or components meet in a manner than results in narrow spaces between the components. Corrosion can also be the result of faulty manufacturing processes that result in microstructures that render an alloy susceptible to corrosion.
Take our Corrosion of Metals course if you’d like to learn more about these corrosion mechanisms, such as why they occur and how to prevent them from occurring. Also, the books Corrosion and Corrosion Control (4th edition) by R.W. Revie and H.H. Uhlig and Corrosion: Understanding the Basics by ASM International are good resources for information about the different corrosion mechanisms.
"A group of us took several courses (Principles of Metallurgy, Metallurgy of Steel, Corrosion of Metals) to become more knowledgeable about the science of metals to avoid problems. For me, the biggest impact of the training was on working with suppliers. I feel more confident asking questions and I now know the suppliers which know their stuff and which ones don’t. And it was great being able to get the training when it was convenient for me."Sam Bloodgood, VP Process Improvement, Hydraforce, Inc.
"I oversee several operations, including steel heat treating and laser welding. However, my background was in the construction materials industry. Principles of Metallurgy gave me the knowledge to have meaningful discussions with my engineers and be able to ask them better questions."Tom Parkman, Plant Manager, Simonds International.
“Principles of Metallurgy exceeded my expectations. The content was straightforward enough not to be burdensome, yet deep enough to provide a practical review of fundamental principles. I recommend this course to any engineer or technical person who has been out of school and working in industry for several years, but not necessarily having been focused on metallurgy.”Andy Jacobs, Staff Engineer, DePuy Orthopaedics, Inc.
“The Principles of Metallurgy course is broke up into convenient length modules that can be fit into the busiest schedule. The course is a good review for engineers who had a materials class ten or more years ago.”Paul Flury B/E Aerospace
“This is an excellent course (Metallurgy of Steel Heat Treating) for learning basic heat treating practices. The course introduces and covers a broad range of processes. I would recommend it for anyone in the steel business.”Jim Marks, Magellan Corporation
“This course has given me more confidence in my job and given me a better understanding of some of the heat treatments used in the business.”Mark Winter, Abbey Forged Products
Why Industrial Metallurgists?
- Practical, up-to-date content.
- Metallurgy expertise and 20+ years of experience.
- Courses designed for non-metallurgists.
- Training accessible from anywhere with internet access.
- Engaging content.
- Convenience. Learn when it suits your schedule.
- Avoid being overwhelmed with too much information at one time. Set your own pace.