{"id":82628,"date":"2021-04-01T00:00:00","date_gmt":"2021-04-01T00:00:00","guid":{"rendered":"https:\/\/www.corrosionpedia.com\/2021\/04\/01\/intergranular-corrosion-what-it-is-and-how-to-stop-it"},"modified":"2022-01-26T20:56:43","modified_gmt":"2023-12-09T19:06:02","slug":"intergranular-corrosion-what-it-is-and-how-to-stop-it","status":"publish","type":"post","link":"https:\/\/www.corrosionpedia.com\/intergranular-corrosion-what-it-is-and-how-to-stop-it\/2\/7229","title":{"rendered":"Intergranular Corrosion: What It Is and How To Stop It"},"content":{"rendered":"

There are many types of corrosion damage, such uniform corrosion<\/a> and pitting corrosion<\/a> that people can readily see with the naked eye. However, some corrosion damage is not visible while still being detrimental to the structure's or equipment's integrity. This article will take a closer look at one of the less visible corrosion damage types called intergranular corrosion (IGC)<\/a>, with a focus on developing a deeper understanding of how intergranular corrosion occurs, what materials at affected, the types of industries where intergranular corrosion typically occurs, and how to detect and mitigate the damage.<\/p>\n

What is Intergranular Corrosion (IGC)?<\/h2>\n

Intergranular corrosion (IGC), sometimes referred to as intergranular attack (IGA), is a preferential or localized corrosion<\/a> proceeds alone the grain (crystal) boundaries<\/a> or immediately adjacent to the grain boundaries. In contrast, the majority of the grains remain mostly unaffected.<\/p>\n

Although metal loss is minimal, IGC can cause the catastrophic failure of equipment. IGC is a common form of attack on alloys in the presence of corrosive media that results in the loss of strength and ductility<\/a>. One should not mistake IGC with stress corrosion cracking (SCC)<\/a>. SCC requires stresses (residual or applied) to act continuously or cyclically in a corrosive environment producing cracks following an intergranular path<\/a>.<\/p>\n

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Figure 1. Intergranular corrosion attack in austenitic cold rolled stainless steel sheet.<\/strong> (Source: Antkyr, Creative Commons ShareAlike 3.0 Unported (CC BY-SA 3.0<\/a>).)<\/p>\n

How Intergranular Corrosion (IGC) is Formed<\/h2>\n

The ICG localized corrosion at grain boundaries is caused by<\/a> the anodic dissolution<\/a> of areas weakened by the alloying elements, second phase precipitation or regions with isolated alloying or impurity elements.<\/a> The remaining part of the exposed surface typically functions as the cathode<\/a>, and large cathodic areas support the anodic dissolution process.<\/p>\n

The cathode to anode<\/a> ratio is generally greater than one. It depends on factors<\/a> such as the volume fraction and distribution of electrochemically active phases, the distribution of detrimental alloying and impurity elements, and grain size<\/a>.<\/p>\n

The corrosion rate<\/a> is dependent on the dominant corrosion mechanism, and factors such as the diffusion of species to or from the anodic front can govern the dissolution kinetics. A significant characteristic of IGC is the development of a relatively homogeneous and uniform depth of attack. The dissolution of grain boundaries causes the dislodging of grains, often referred to as grain dropping<\/a>. Grain dropping is responsible for most of the weight loss observed after IGC exposure, and corrosion rates can therefore be several orders of magnitude higher than during general corrosion<\/a>.<\/p>\n

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Figure 2. A stainless steel that corroded near a weld's heat affected zone (HAZ).<\/strong> (Source: NASA Corrosion Engineering Laboratory<\/a>.)<\/p>\n

Materials Commonly Affected by Intergranular Corrosion<\/h2>\n

Intergranular corrosion attack is mainly prevalent in certain types of stainless steel<\/a> rather than in carbon steel<\/a>. (Related reading: Why Stainless Steel is Corrosion Resistant<\/a>.) However, the following materials are not excluded from IGC attack.<\/p>\n