When one hears the words “metal” and “corrosion” used together, the most common image that comes to mind is something made of steel or iron covered in orange-red rust. And, that’s perfectly accurate and acceptable, given how commonplace the slow-motion oxidation of iron is in our world. The old metal sign, the fender on the beater car in front of you that looks like it’ll fall apart any moment, or even the kitschy art piece where real or fake corrosion has been applied to give an object “character.” But, corrosion is much more than a simple case of Fe2O3, aka “iron oxide” or, as we all call it, “rust”.
Rust refers only to the corrosion of iron or steel. Other metals can corrode and there are 10 common types of corrosion. While their cause, color and particular process can differ, they are all common in that they will cause the eventual deterioration of the metal involved.
General Attack Corrosion is the most common form. On iron, it is what we call “rust”. It is a chemical or electro-chemical reaction. With iron and many other metals, it’s more than the interaction of the metal with the oxygen in the atmosphere, it requires some level of moisture to initiate the chemical reaction. Other metals corrode, of course. Perhaps most surprisingly is that aluminum is very prone to corrosion, but the aluminum oxide that forms is a very hard material that actually protects the underlying metal from further corrosion, often called a passive oxide film.
Overall, general attack corrosion is responsible for the vast majority of metal lost to corrosion, but at the same time it is considered manageable, as it is the most thoroughly understood and is therefore predictable and frequently preventable.
Localized Corrosion is, as its name suggests, a corrosion that takes place in a particular area of the metal. There are three subtypes: Pitting, Crevice and Filiform:
- Pitting – takes place when one or more small holes form on the surface of the metal due to a mechanical impact or a chemical reaction. This sets up a small electric current with the pit being the anode (generating negative ions) and the remaining metal serving as the cathode in a galvanic reaction. Because of its size, this sort of corrosion is hard to detect. If left untreated it can penetrate the metal causing failure.
- Crevice – like pitting, it occurs in a specific location and frequently occurs in an area in constant contact with moisture, such as under a gasket, washer, between layers of metal or the like. If conditions create an acidic environment or if oxygen is depleted within the area, then corrosion is likely to occur.
- Filiform – Perhaps the hardest to initially detect, it occurs under painted or plated surfaces when a failure of the coating allows moisture to accumulate. The corrosion begins at the point of the defect and expands into the surrounding metal, often leading to a blistering of the paint or plating.
Galvanic Corrosion, also known as dissimilar metal corrosion, is much like the pitting electrochemical reaction, above, but on a much larger scale. It happens when two dissimilar metals, such as brass and iron, are in direct contact and exposed to an electrolyte which could be as simple as water. The two items form a galvanic couple where one metal serves as the anode corroding as its ions flow to the other metal which acts as the cathode. Ultimately the anode part will deteriorate faster while the cathodic part will actually corrode at a much slower rate. Galvanic corrosion is particular risk for ocean vessels and a current is induced into the hull to counteract this process and protect the hull.
In Environmental Cracking, a chemical reaction combines with one of a number of environmental conditions to corrode the metal. Temperature and stress can serve to accelerate the deterioration of metal in four principle situations:
- Stress Corrosion Cracking (SCC) is the growth of a crack formation within the metal while in a corrosive environment. Certain alloys are likely to undergo SCC in very specific chemical environments (such as chloride attack) which are otherwise only slightly corrosive. It can lead to unexpected and sudden failure of metal alloys under stress and usually at elevated temperatures.
- Corrosion Fatigue also requires a corrosive environment, with the addition of the mechanical degradation due to cyclic loading, otherwise known as alternating stress. This is of particular concern in airframes.
- Hydrogen-Induced Cracking (HIC) – also known as hydrogen embrittlement (HE) and hydrogen assisted cracking (HAC) is a complex process that is not completely understood because of the variety and complexity of factors that can cause it. Steel can become brittle when exposed to hydrogen at hot temperatures as the element combines with the carbon in the metal to form methane within its voids. Copper alloys which contain oxygen inclusions will similarly absorb the hydrogen, which then binds with the oxygen to form water. This water forms pressurized bubbles of steam at the grain boundaries and cause the grains to be forced away from each other. Similarly, vanadium, nickel and titanium alloys can also absorb hydrogen which can cause expansion and damage to their crystal structure. However, austenitic stainless steel is not subject to this embrittlement.
- Liquid Metal Embrittlement is caused by exposure to particular liquid metals. Both mercury and gallium can cause this failure with aluminum being particularly susceptible to mercury.
Fretting Corrosion is similar to Stress Corrosion Cracking in that a mechanical force is required for it to take place. In the case of fretting corrosion, the item involved is subject to repeated wearing, weight and/or vibration on a rough surface. An example of where this can occur is within a ball bearing. The mechanical damage also further increases the surface roughness, making it even more susceptible to further damage. It can also potentially cause pitting of one or both surfaces. The newly exposed metal, along with any debris that was formed by the initial damage will oxidize. In some situations, the oxidized metal is even harder than the original material, but regardless, the oxidized debris can further act as an abrasive upon the surface, speeding up the deterioration of the metal.
High-Temperature Corrosion generally takes place in machines which burn fuel. When an engine or gas turbine burns fuel containing impurities, it can leave behind a residue that is very corrosive at the high temperatures in which it operates. One common example is marine fuel oil, which naturally contains vanadium. Another example is sulfur in diesel fuel, which while the sulfur improved fuel economy and the lubricity of the fuel, it is blamed for a number of environmental and health concerns. The sulfate compounds formed when the fuel is burned are even capable of affecting stainless steel. Additionally, oxygen, sulfides, steam and carbon can also attack certain metals at high temperature.
Flow-Assisted Corrosion (FAC) takes place when the protective layer of a metal, such as the passive layer of stainless steel is eroded by gas or water, exposing the underlying metal to chemical or mechanical corrosion. It is particularly common in carbon steel piping carrying ultra-pure deoxygenated water or wet steam.
- Impingement Corrosion involves high velocity fluid flowing against a solid surface creating a pattern of localized corrosion.
- Erosion-Assisted Corrosion takes place when the liquid contains some abrasive solid particles, such as a slurry.
- Cavitation results from a rapid change in pressure of the liquid causing it to form bubbles. These bubbles collapse and create shock waves which can be damaging to nearby metal surfaces. This frequently occurs in pump impellers and tight bends in pipes.
Intergranular Corrosion often occurs in metals with impurities. These impurities tend to occur near grain boundaries. Grain boundaries are two-dimensional defects within the crystal structure of the metal and they tend to decrease both its electrical and thermal properties making these areas more vulnerable to chemical or electrochemical reactions than the bulk metal.
De-Alloying, as its name implies, takes place in metal alloys. In this case, only one of the metals forming the alloy corrodes while the other metal is left mostly intact. Zinc, carbon, nickel and cobalt are four of the more common elements to leach from their alloys. A chemical reaction or welding is generally required, although zinc and nickel can be removed from their alloys with copper after extended exposure to water. The remaining copper structure is porous and prone to failure.
As can be seen, corrosion takes on a number of forms. However, even with diverse causes and methods, they are all a serious threat to the integrity of whatever metal structure they take place upon. Whether the corrosion is visible on the surface, or if it requires specialized equipment to detect, having a plan of protection and prevention of corrosion is absolutely essential.