Testing For Intergranular Corrosion and How To Stop It

Intergranular corrosion can lead to catastrophic failure in equipment if proper material and treatment haven't been used during the fabrication stage.

There is considerable corrosion damage, such as uniform corrosion and pitting corrosion, that people can easily see with the naked eye. However, some corrosion damage is not visible while still being harmful to the structure or equipment integrity. This article will take a better take a look at one of the less visible corrosion damage types called intergranular corrosion (IGC), with a concentrate on developing a much deeper understanding of how intergranular corrosion happens, what materials are impacted, the kinds of markets where intergranular corrosion generally happens, and how to spot and reduce the damage.

What is Intergranular Corrosion (IGC)?

Intergranular corrosion (IGC), sometimes described as an intergranular attack (IGA), is a preferential or localized corrosion profit alone the grain (crystal) borders or right away adjacent to the grain borders. On the other hand, most of the grains stay primarily untouched.

Although metal loss is very little, IGC can trigger the catastrophic failure of equipment. IGC is a common form of attack on alloys in the presence of destructive media that leads to the loss of strength and flexibility. One should not error IGC with stress corrosion cracking (SCC). SCC needs stresses (recurring or used) to act constantly or cyclically in a corrosive environment producing cracks following an intergranular course.

Intergranular corrosion attack in austenitic cold-rolled stainless steel sheet.

Figure 1. Intergranular corrosion attack in austenitic cold-rolled stainless steel sheet. (Source: Antkyr, Creative Commons ShareAlike 3.0 Unported (CC BY-SA 3.0).).

How Intergranular Corrosion (IGC) is Formed.

The ICG localized corrosion at grain limits is caused by the anodic dissolution of areas deteriorated by the alloying elements, 2nd stage rainfall, or areas with isolated alloying or impurity components. The remaining part of the exposed surface usually functions as the cathode, and large cathodic areas support the anodic dissolution process.

The cathode to anode ratio is generally greater than one. It depends on the volume fraction and distribution of electrochemically active phases, the circulation of harmful alloying and impurity elements, and grain size.

The corrosion rate depends on the dominant corrosion system, and aspects such as the diffusion of species to or from the anodic front can govern the dissolution kinetics. An important characteristic of IGC is the advancement of a fairly homogeneous and uniform depth of attack. The dissolution of grain limits causes the dislodging of grains, typically described as grain dropping. Grain dropping is responsible for most of the weight loss observed after IGC direct exposure, and corrosion rates can therefore be numerous orders of magnitude higher than during basic corrosion.

Figure 2. Stainless steel that corroded near a weld's heat-affected zone (HAZ). (Source: NASA Corrosion Engineering Laboratory.).

Materials Commonly Affected by Intergranular Corrosion.

Intergranular corrosion attack is mainly widespread in specific types of stainless steel rather than in carbon steel. (Related reading: Why Stainless Steel is Corrosion Resistant.) Nevertheless, the following materials are not left out from the IGC attack.

Unstabilized austenitic stainless-steel grades 304 and 316 used in chemical plants are prone to IGC attack when utilized in the sensitized stage. The chromium carbide precipitation brings on the sensitization at the grain boundaries in a zone adjacent to welds, where the temperature level has been between 500 - 800 ° C (932- 1472 ° F). ( For more on this subject, read How Hot Shortness and Welding Affect Corrosion in Metals.).

Nickel-copper alloys (Alloy 400, UNS N04400) are prone to IGC attack when exposed to particular types of hydrofluoric acid and chromic acid options.

IGC attack can occur in nickel-molybdenum alloys (Alloy B, UNS N10001) exposed to hot hydrochloric and sulfuric acid due to the rainfall of molybdenum-rich constituents.

Nickel-chromium alloys such as Alloy 600 are prone to IGC attack. Therefore, it is not intended for use in service in corrosive environments.

Aluminum grades 2024 and 7075 are prone to IGC attack since CuAl2 precipitates at the grain borders that act as cathodes, accelerating the depleted zone adjacent to the grain boundary. Additionally, aluminum grades 5083 and 7030 are likewise susceptible to IGC attack.

Zinc (Zn) of high purity is not vulnerable to IGC. However, aluminum as an alloying component or impurities in the zinc alloy could trigger an IGC attack.

Intergranular Attack of Austenitic Stainless Steels.

With austenitic stainless-steels, the intergranular attack normally performs chromium carbide precipitation (Cr23C6) at grain borders, which produces a narrow zone of chromium depletion at the grain limit. This condition is described as sensitization (Figure 3). Sensitization involves the rainfall of chromium carbides at grain boundaries, which leads to a narrow zone of chromium deficiency at the grain boundary.

Figure 3.

The chromium is the primary alloying element that makes stainless steel corrosion-resistant; the chromium-depleted regions are prone to preferential corrosion attack. It is believed that this takes place since the chromium content immediately adjacent to the carbide may be listed below that required for the stainless-steel alloy. If the carbides form a continuous network on the grain boundary, corrosion can produce separation or space at the border and possible grain dropping or loss.

Methods to Detect Intergranular Corrosion.

Usually, the IGC proceeds along grain borders and is hard to spot with the naked eye or any other non-destructive evaluation strategy. Nevertheless, the material can be tested for resistance to IGC before fabrication of the equipment with particular lab approaches, such as the Huey test (which utilizes a nitric solution) or the Strauss test to identify the susceptibility of stainless-steel to intergranular corrosion. The Streicher test can also be used, which is based on a quantitative weight loss decision. Furthermore, IGC splitting can be seen when a sample from the failed location is metallographically ready and examined under a scanning electron microscopic lens (SEM).

Mitigation Methods to Prevent IGC Attack of Austenitic Nickel-Chromium Stainless Steel.

Conducting correct annealing and quenching treatments at the fabrication store or mill will lower the vulnerability of stainless steel and nickel-rich chromium-bearing alloys to IGC. When these treatments are successfully performed, liquified chromium carbides, nitrides, and molybdenum carbides, and their pre-precipitation kinds, keep them in solution throughout the quenching.

In ferritic stainless steels (AISI Type 430, Type 446), the diffusion rate of carbon is so fantastic that the rainfall of chromium carbides can not be avoided, even with quick water quenches from high-temperature annealing treatments. However, the rate of chromium diffusion in these alloys is likewise high. It is possible to bring back the chromium-depleted zones surrounding the chromium carbide speeds up with heat treatments near 816 ° C( 1,500 ° F ). The outcome is a microstructure that contains vast quantities of carbide residue, which is immune to IGC.

The heat treatment of pipeline welds to prevent intergranular corrosion.

Figure 4. The heat treatment of pipeline welds to prevent intergranular corrosion. (Source: Berkut34|Dreamstime.com).

When stainless alloys are welded, the development of chromium carbides and nitrides can be avoided often by decreasing the carbon and nitrogen content. The introduction of the argon-oxygen decarburization procedure, vacuum melting, and consumable arc remelting have significantly influenced avoiding chromium carbides and nitride development in the alloys AISI Type 304L, Type 316L, Alloys C-276, and C-4, and the Fe-29% Cr-4% Mo.

The formation of chromium carbides in stainless steel can be avoided by including the components titanium (Ti) or Niobium (Nb). (Related reading: The Role of Chromium in Intergranular Corrosion.) These components integrate with carbon and lower its concentration such that chromium carbides are not formed during direct exposure in the sensitizing variety of temperature levels during welding and tension alleviating, and even under operating conditions. These are called supported alloys, and they are AISI Type 321 (Ti), AISI Type 347 (Nb), Alloy 20Cb-3( Nb), Alloy 625 (Nb), and Alloy 825 (Ti).

Various sizes of weldments and other welding strategies (such as lower heat inputs) can reduce the degree of sensitization. However, it isn't simple to preserve definite control to make this approach usually relevant.

Industries that are Often Affected by Intergranular Corrosion.

IGC can occur on any devices produced from austenitic stainless steel, nickel-copper alloy, nickel-molybdenum alloy, nickel-chromium alloy, aluminum alloy, and zinc alloy in any market where the best conditions exist, which indicates that if the material has not undergone appropriate heat treatment and consists of a higher carbon content (C > 0.03%), then it is vulnerable.

Intergranular corrosion can result in catastrophic failure in most process equipment if correct material and appropriate heat treatment haven't been utilized during the fabrication stage. The loss of cross-section thickness and the intro of fractures can have severe repercussions for pressure containment applications.


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