Different Types of Localized Metal Corrosion
This article is intends to discuss:
- Localized corrosion occurs at intense attack on the confined area of the metal surface
- Several types of corrosion are galvanic, crevice, pitting and intergranular
- All these types of corrosion can be reduced using anti-corrosion protective coatings
What is Localized Metal Corrosion?
- Localized corrosion refers to the type of corrosion that occurs at local areas on the metal surface. It penetrates the metal at a higher rate due to the environmental effects or the inherent properties of the material. Several types of localized corrosion that easily to be distinguished are galvanic, crevice, pitting and intergranular which will be discussed in detail in the following section.
This form of corrosion is usually observed when two dissimilar metals having different electrical potentials are in contact, either through an electrolyte or physical contact. The less noble metal (lower electrical potential) relative to the other acts as the anode, while the opposing metal acts as the cathode. This electrical potential difference in turns drive the corrosion reaction and causes the less noble metal to start failing.
The 3 requirements for galvanic corrosion to occur are:
- Two dissimilar metals having different electric potentials
- Two dissimilar metals in contact with one another
- Two dissimilar metals exposed to the same corrosive environment (e.g. containing water & oxygen)
A classic example of galvanic corrosion due to different electric potential between the bolts & plate.
[Image by D3j4vu from http://en.wikipedia.org]
In most cases, galvanic corrosion can be easily prevented with proper selection of materials during the design stage of a structure or system. However, metallic coatings are typically used to protect bimetallic systems against this form of corrosion. Zinc is a common example of metallic coatings which can act as a barrier or sacrificial anode to corrosion, such as Dri-Gard ZN 90.
Crevice corrosion is resulted when there is a difference in ion concentration between two touching materials & it usually occurs in stagnant micro-spaces. These trapped areas include joints, rivet heads, bolt heads, gaskets and etc. where moisture enters easily but oxygen circulation is prohibited. Anodic imbalance is created by the combination of reduced oxygen content in the crevice area relative to the surrounding environment, thus resulting in a highly corrosive microenvironment.
Illustration of the conditions that causes crevice corrosion
The oxygen concentration inside the crevice is depleted, hence making the surface area a small anode compared to the outside of a crevice. This creates anodic imbalance. The accumulation of acidic hydrolyzed salts results in a highly corrosive microenvironment in the crevice area.
Passive materials such as aluminum alloys and stainless steels have a greater susceptibility to crevice corrosion. Copper alloys can also experience this form of corrosion in seawater environment. While protective coatings can be used to protect the metal from corrosion, it is important, however, to implement regular cleaning schedule to remove debris that can be built up.
Pitting is one of the most extreme localized form of corrosion that is formed by small holes or pits due to the attack of corrosive medium. Pitting corrosion can be one of the most destructive and dangerous forms of corrosion as it is difficult to detect and can happen at a very fast rate. Once a pit has initiated, it can penetrate from the surface and coalesce underneath which weaken the metal and results in failure eventually.
[Image from Funtay/Shutterstock.com]
This can happen naturally on a surface as a local outbreak, or it can also be caused by non-uniformities in the metal structure itself. If anti-corrosion coatings are not applied properly, pitting corrosion is likely to attack areas where the coating has failed.
The factors responsible for pitting corrosion are:
- Chemical or mechanical damage to the protective coating film
- Thinning or breakdown of protective coating film
- Inadequate protective coating application
- Non-uniformities or local differences in composition of metal structure
Intergranular corrosion is associated with impurities present at grain boundaries which are formed during solidification of an alloy. This form of corrosion attacks the interior of metals along with the grains.
It is well known for metals that have been soaked under high temperatures where carbide precipitation (a chemical reaction that occurs when metal is subjected to high heat) can occur.
Illustration of granular corrosion
[Image from Unified Alloys]
During these reactions, carbon reacts with chromium forming carbides, causing chromium content to fall. However, a minimum of 12% of chromium in metal is needed to ensure passivation. When the metal has a chromium content lower than 12%, intergranular corrosion can easily start.
Prevention of Localized Metal Corrosion
Selection of proper protective coating that is suitable for this temperature can be a solution or prevention to this form of corrosion.
Click here for more information on metal corrosion and Dritech’s range of anti-corrosion metal coatings.
Please contact us for more information on appropriate protective coating systems for your structures.
by Oh Hui Xuan
[15 May 2022]
15 Dec 2022