How to Prevent Pitting Corrosion
On the surface, pitting corrosion may seem insignificant. Underneath the surface, however, pitting on the part can quickly deteriorate the metal material. This article will briefly describe the nature of pitting corrosion, how to identify it, and four ways manufacturers can prevent pitting corrosion from occurring in machined parts.
Pitting is one of the most common forms of corrosion. It is a form of localized corrosion that affects metals with small defects in otherwise passivated surfaces, including stainless steel, aluminum, and aluminum alloys. Pitting corrosion is particularly insidious because the damage may appear insignificant on the surface, but be extensive underneath the surface, where it is not visible. The hidden degradation can compromise a part’s strength and integrity. For this reason, pitting on the part is often considered to be one the most dangerous forms of corrosion. This article will briefly describe the nature of pitting corrosion, how to identify it, and four ways manufacturers can prevent pitting corrosion from occurring in machined parts.
Corrosion is the deterioration of a metal due to electrochemical oxidation-reduction reactions that occur between the atoms at the surface of a metallic part and the atoms in the service environment. Pitting is a localized type of corrosion characterized by tiny holes or “pits” on the surface of a part. Metals that naturally form thin, protective oxide coatings - like stainless steel, aluminum alloys, and others - are the ones most susceptible to pitting corrosion.
Pits can result from poor or damaged surface finishes - whether, from direct machining, an inadequate finishing process, or damage after the part is manufactured. Some materials, like aluminum and stainless steel, naturally form oxide films on their surfaces that protect the material under the surface when the part is exposed to the environment. Under ideal conditions, this film should cover the whole part, however, part damage and foreign contaminants can prevent this film from adequately protecting the part. As a result, the vulnerable metal beneath the surface of a part is left exposed and untreated to the atmosphere. The metal beneath the surface reacts with the atmosphere - leading to corrosion. If there are several pits on a part, the damage can be far more extensive. In some cases, pits are merely an aesthetic issue. However, in structural or load-bearing parts, the loss of wall thickness and the introduction of stress concentrations can severely limit the life of a part and can pose significant safety risks.
Visual examination of a part is the primary way of identifying the presence of pitting corrosion. As corrosion proceeds, byproducts like rust or carbonate begin to form at the edges of pits, making them easily identifiable. The problem is that once these pits become noticeable, damage underneath the surface has more than likely occurred already. While the density of pits present on a part is concerning, the real danger lies in the depth and expansiveness of the pits - the deeper and more expansive the pit, the more material that is exposed to the atmosphere. Other methods can be used to test for the presence of pits and can help describe pits further (i.e. trough or sideway pits, shallow or deep, wide or narrow, etc.), including:
Methods of pitting corrosion prevention consist of selecting appropriate material, controlling environmental conditions, polishing surfaces, and applying protective coatings to a part. These are further described below:
The first and most important step that can be taken to prevent pitting corrosion is to select an appropriate material. This means choosing metals that have adequate resistance to pitting corrosion in the intended service environment, while still being able to meet the application’s requirements for mechanical, thermal, or electrical properties. This reduces the likelihood of needing to employ any additional corrosion prevention methods.
Since corrosion is caused by reactions between a metal and the surrounding environment, another way to prevent pitting corrosion is to control the service environment, to the extent possible. Of course, for parts that are to be used outside, it’s nearly impossible to control all environmental factors. For parts used inside, however, controlling the environment can be as easy as keeping a part clean, limiting water exposure, or regulating temperature as to keep the environment below the critical pitting temperature (CPT) of the material. Additionally, humidity can be regulated to limit or prevent chemical reactions from occurring. In some cases, manufacturers must get more creative and regulate factors such as pH levels or chloride concentrations. Regulating environmental factors can go a long way towards preventing pitting on the parts.
Protective coatings, like painting or powder coating, are an excellent way to inhibit corrosion since these coatings act as a physical barrier between the metallic substrate and the surrounding environment. For manufacturers, it’s important to properly prepare metallic surfaces so that paint and powder coatings can adhere to the part.
Although paint and powder coating can both offer corrosion protection, their use is not a perfect solution. These coatings will eventually wear out - leading to corrosion to occur. Therefore, it’s important for manufacturers to consider the environment where the part will be used in.
Passivation is a process where metals are treated and coated with materials that reduce the chemical reactivity of their surface. Galvanizing is a type of passivation and metal plating process that provides cathodic protection. In this process, a metallic substrate, like cast iron or hot-rolled steel, is coated with molten zinc. It inhibits corrosion because zinc is more active (anodic) than ferrous substrates. Therefore, the coated layer of zinc acts as a sacrificial anode - meaning the electrochemical reactions associated with corrosion occur within the zinc, not within the ferrous substrate. As long as zinc is present on the surface near the localized corrosion, the substrate will be protected. Another passivation process similar to galvanizing is anodizing. Anodizing is more suited for aluminum and other non-ferrous metals.
On the surface, pitting corrosion may seem insignificant and may be deemed only an aesthetic issue. Underneath the surface, however, pitting on the part can quickly deteriorate the metal material. The strength of structural or load-bearing components - like in a bridge or building - can be compromised. Therefore, pitting presents an enormous risk to human life. Thankfully, there are steps manufacturers can take to prevent pitting corrosion from occurring - from selecting an appropriate material to applying protective coatings to parts.
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