types of intergranular corrosion

Intergranular Corrosion The preferential attack at, or adjacent to, the grain boundaries of a metal. What causes intergranular Anisotropic alloys, where extrusion or heavy working leads to formation of long, flat grains, are especially prone to intergranular corrosion. | Founder & Director, Materials Info Consultancy Private Limited. The cracking of metals caused by the combined effects of cyclic stress and corrosion. High-strength aluminium alloys such as the Duralumin-type alloys (Al-Cu) which depend upon precipitated phases for strengthening are susceptible to intergranular corrosion following sensitization at temperatures of about 120C. Sensitization involves the precipitation of chromium carbides at grain boundaries, which results in a narrow zone of chromium depletion at the grain boundary. Corrosion can usually be placed in to one or more of the following visual forms: General or Uniform Corrosion. Knife-Line Attack The more commonly seen transgranular fracture, occurs when the crack grows through the material grains.As an analogy, in a wall of bricks, intergranular fracture would correspond to a fracture . Nickel-rich alloys such as Inconel 600 and Incoloy 800 show similar susceptibility. The tests require etching with chemicals that reveal the presence of intermetallic particles, sometimes combined with Charpy V-Notch and other mechanical testing. In any case, here we are going to describe the five types of corrosion found in aviation. These carbides build up next to the weld bead where they cannot diffuse due to rapid cooling of the weld metal. However, it's not particularly severe, and it rarely affects any item to the point of rendering it useless. AI Cu, AI mg Si, Al Mg and Al Zn Mg alloys are sensitive to intergranular corrosion. boundary precipitation, notably chromium carbides in stainless steels, is a Intergranular corrosion is sometimes also called "intercrystalline corrosion" or "interdendritic corrosion". Intergranular corrosion (IGC) is a selective attack in the vicinity of the grain boundaries of a stainless steel. SCC is a type of intergranular attack corrosion that occurs at the grain boundaries under tensile stress. Aluminium-based alloys may be sensitive to intergranular corrosion if there are layers of materials acting as anodes between the aluminium-rich crystals. Unfortunately, the method is less than ideal for treating large assemblies. Unlike many forms of stainless steel corrosion, intergranular corrosion occurs at a microscopic level -- impacting the very structure of the metal without always showing a sign of attack on the surface. This type of corrosion is called intergranular corrosion, and most metals and alloys may exhibit intergranular corrosion in specific corrosive media. The chemical composition of common metallurgical materials includes nitrides, carbides, and intermetallic phases that . Types of corrosion normally found on aircraft include: Uniform surface attack. This means the metal itself is composed of grains of various sizes. Galvanic Unified Alloys will not be responsible for the accuracy or currency of any of the information contained herein. Intergranular corrosion: This type of corrosion affects the grain boundaries of an alloy, so it's often related to a lack of uniformity in alloy structure. ASTM International offers standards to help to determine the susceptibility of a given alloy or component to intergranular attack as follows: ASTM A262 - 15: Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels, ASTM A763 - 15: Standard Practices for Detecting Susceptibility to Intergranular Attack in Ferritic Stainless Steels. MIC These boundaries can be more vulnerable to corrosion than the bulk of the metal. For example, a high-temperature solution heat treatment, commonly termed solution-annealing, quench-annealing or solution-quenching, has been used. Where these grains meet are called the grain boundaries. Exfoliation corrosion is a special type of intergranular corrosion found in aluminium alloys that have marked directional structures. Much like stress corrosion cracking, this can occur with little to no visual indication of a corrosive attack.. piping, tubing, flanges, fittings, and other stainless steel products. Why They Resist Corrosion and How They Fail. The type of corrosion present tells a lot about the corrosion mechanisms involved and possible methods to control or alleviate the corrosion problem. Here is a brief overview of some common types of corrosion: Galvanic corrosion is the most common and impactful form of corrosion. Intergranular corrosion induced by environmental stresses is termed stress corrosion cracking. It is possible to restore the chromium-depleted zones surrounding the chromium carbide precipitates with heat treatments near 816C (1,500F). Privacy Policy|Terms of UseMembership Terms of Content Use, The Association for Materials Protection and Performance. This is caused by local chemical differences such as the chrome-depleted regions of an austenitic stainless steel. Grain boundaries that are rich in chromium elements will precipitate lead. Intergranular Corrosion. . Intergranular corrosion (IGC), sometimes referred to as intergranular attack (IGA), is a preferential or localized corrosion proceeds alone the grain (crystal) boundaries or immediately adjacent to the grain boundaries. In a corrosive atmosphere, the grain interfaces of these sensitized alloys become very reactive and intergranular corrosion results. By: Raghvendra Gopal . The microstructure of metals and alloys is made up of grains, separated by grain boundaries. Conducting proper annealing and quenching treatments at the fabrication shop or mill will reduce the susceptibility of stainless steel and nickel-rich chromium-bearing alloys to IGC. However, the following materials are not excluded from IGC attack. 1. Chromium carbides can be precipitated if the stainless steel is sensitized in the temperature range 550-850C (1020-1560F), for example during heat treatment or welding. In any case the mechanical properties of the structure will be seriously affected. A significant characteristic of IGC is the development of a relatively homogeneous and uniform depth of attack. Intergranular Corrosion This infrequent type of corrosion preferentially attacks a material at the grain boundaries. in composition, such as coring commonly encountered in alloy castings. When stainless alloys are welded, the formation of chromium carbides and nitrides can be prevented in many cases by reducing the carbon and nitrogen content. Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity. It often occurs due to impurities in the metal, which tend to be present in higher contents near grain boundaries. As the name suggests, this is a form of corrosive attack This website uses cookies to ensure you get the best experience on our website. This type of corrosion occurs at a granular level near the grain boundaries of the metal. This makes the boundaries very vulnerable to corrosion attacks in various electrolytes. Corrosion and Its Prevention (5-day module) (Source: Antkyr, Creative Commons ShareAlike 3.0 Unported (CC BY-SA 3.0).). Optical and SEM microscopy studies were carried out to investigate the nature of IGC at 700 C with increasing time (15, 30, 60, 180, 360, 720, 1440 min) according to ASTM standard 262 A. Quantitative analysis was performed . Intergranular corrosion is sometimes also called "intercrystalline transgranular corrosion.). This type of corrosion is especially prevalent in some stainless steels. It results from chromium depletion in . The galvanic potential of the grain-boundary areas of an alloy is anodic to that of the grain Usually the IGC proceeds along grain boundaries and is hard to detect with the naked eye or any other non-destructive inspection technique. - Crevice corrosion: it is a form of local corrosion, which may occur in the crevice where the solution stagnates or on the shielded surface. 3,562,781. Although metal loss is minimal, IGC can cause the catastrophic failure of equipment. Intergranular Corrosion. This article will take a closer look at one of the less visible corrosion damage types called intergranular corrosion (IGC), 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. By clicking sign up, you agree to receive emails from Corrosionpedia and agree to our Terms of Use & Privacy Policy. Such sensitization of austenitic stainless steels can readily occur because of temperature service requirements, as in steam generators, or as a result of subsequent welding of the formed structure. A four-cylinder spark-ignition engine has a compression ratio of 10.5, and each cylinder has a maximum volume of 0.4 L. At the beginning of the compression pro- cess, the air is at 98 kPa and $37^{\circ} \mathrm{C},$ and the maximum temperature in the cycle is 2100 K. Assuming the engine to operate on the ideal Otto cycle, determine (a ) the amount of heat supplied per cylinder, (b) the . Weld Decay Alloys that are elongated and flattened from heavy work and extrusion have a high risk of this type of corrosion. Conclusion. Potential Solutions 316/316L Stainless Steel Galvanic Corrosion in the Presence of an Electrolyte Intergranular corrosion can be evidenced as a form of filiform corrosion if not treated. Exfoliation Corrosion Exfoliation corrosion is intergranular corrosion that is most often associated with strong aluminum alloys. Why They Resist Corrosion and How They Fail The samples have been polished and etched before taking the photos, and the sensitized areas show as wide, dark lines where the etching fluid has caused corrosion. Copper-based alloys become sensitive when depletion of copper content in the grain boundaries occurs. These carbides build up next to the weld bead where they cannot diffuse due to rapid cooling of the weld metal. Intergranular corrosion (IGC), sometimes referred to as intergranular attack (IGA), is a preferential or localized corrosion proceeds alone the grain (crystal) boundaries or immediately adjacent to the grain boundaries. | Principal/Chief Technical Officer, Corr-Met Inspection & Consulting Inc. Dennis Jayasinghe is a Senior Corrosion Engineer with over 15 years of combined experience in industry and research. This passive film protects the metal from corrosive environments. The The corrosive attack is restricted to the heat affected zone ().Positive identification of this type of corrosion usually requires microstructure examination under a . Some of the eight forms of corrosion are unique, but all of them are more or less interrelated. Intergranular Corrosion. Knifeline attack impacts steels stabilized by niobium, such as 347 stainless steel. corrosion? In a galvanic cell (bimetallic couple), the more active metal (anode) corrodes and the more noble metal (cathode) is . A classical case is when chromium carbide is precipitated at the grain boundaries. Fatigue Mark Peter Staiger. It is form of surface corrosion preferentially along the grain boundaries of metal. It is often only noticeable once it has progressed to exfoliation corrosion, which is identifiable by the lifting or flaking of the metal at the surface. Intergranular corrosion is generally considered to be caused by the segregation of impurities at the grain boundaries or by enrichment or depletion of one of the alloying elements in the grain boundary areas. The Streicher test can also be used, which is based on a quantitative weight loss determination. Structures made of such steels have to be heated in a whole to about 1065C (1950F), when the chromium carbide dissolves and niobium carbide forms. By clicking sign up, you agree to receive emails from Corrosionpedia and agree to our Terms of Use and Privacy Policy. Grain Intergranular corrosion is a chemical or electrochemical attack on the grain boundaries of a metal. . All rights reserved. Intergranular corrosion attack is mainly prevalent in certain types of stainless steel rather than in carbon steel. stainless steel. One is oxidizing or weak oxidizing medium, and the other is strong oxidizing medium, such as concentrated nitric acid. Intergranular Corrosion This type of corrosion is an attack along the grain boundaries of an alloy and commonly results from a lack of uniformity in the alloy structure. to the unaffected grains. OnePetro (4) Sensitization refers to the precipitation of carbides at grain boundaries in a stainless steel making it susceptible to intergranular corrosion. Stress Corrosion. This guide will discuss what intergranular corrosion is, when it occurs, and best practices to avoid it or minimize the damage caused by an intergranular attack. Welding, improper heat treatments, and exposure to temperatures between 425C and 870C (887F to 1598F) are required to initiate an intergranular attack. It is also the most benign as the extent of the attack is relatively easily judged, and the resul. Figure 4. There are many different types of corrosion that are visible to the naked eye: uniform corrosion, pitting, crevice corrosion, filiform corrosion, galvanic corrosion, environmental cracking, and fretting corrosion, to name a few. Intergranular corrosion Intergranular corrosion is an attack along the grain boundaries of a material. Surface corrosion: a uniform attack where a chemical or electrochemical reaction deteriorates the entire metal surface; Exfoliation corrosion: a type of intergranular attack that removes layers of the metal in a "peeling" fashion; Galvanic/dissimilar metal corrosion: two different metals in a corrosive electrolyte form a . Thus in certain aluminium alloys, small amounts of iron have been shown to segregate in the grain boundaries and cause intergranular corrosion. We, at Applied Technical Services, offer intergranular corrosion testing to ensure the structural integrity of the tested metal remains satisfactory during the service life of metals and alloys. The attack is usually related to the segregation of specific elements or the formation of a compound in the boundary. corrosion" or "interdendritic corrosion". requires microstructure examination under a microscopy although sometimes it 3. The grain interiors then function as cathodic cells, and the intergranular corrosion begins. Weld decay is a form of intergranular corrosion, usually of stainless steels or certain nickel-base alloys, that occurs as the result of sensitization in the heat-affected zone during the welding operation. However, it isn't easy to maintain definite control to make this approach generally applicable. Intergranular corrosion can lead to catastrophic failure in most process equipment if correct material and proper heat treatment haven't been used during the fabrication stage. Fortunately, an intergranular attack requires specific circumstances to occur and, in some cases, the damage is reversible.. Intergranular corrosion can lead to catastrophic failure in most process equipment if correct material and proper heat treatment haven't been used during the fabrication stage. Exfoliation corrosion is visible evidence of intergranular corrosion and most often seen on extruded sections where grain thickness is less than in rolled form. We create world-leading educational content about corrosion and how to preserve the integrity of the worlds infrastructure and assets. Intergranular corrosion (also known as weld decay) affects stainless steel at the structural level and may not show visible signs of damage until corrosion has significantly progressed. High strength aluminium alloys, especially when extruded or otherwise subjected to high degree of working, can undergo exfoliation corrosion (metallurgy), where the corrosion products build up between the flat, elongated grains and separate them, resulting in lifting or leafing effect and often propagating from edges of the material through its entire structure. Additionally, he is an expert in solving quality-related issues of steel, stainless steel and galvanizing. The grain boundaries get attacked and become depleted of chromium, making them vulnerable to corrosion even if they are usually corrosion-resistant metals. Stress Corrosion/Cracking Let's see them in detail. Exfoliation corrosion is a severe type of intergranular corrosion that raises surface grains from metal by forming corrosion products at grain boundaries under the surface. Show abstract. Prevention, Stainless Steels and Alloys: This kind of corrosion can occur in high-purity aluminum in hydrochloric acid and high-temperature water. Introduction. Stainless steels and weld decay sensitization are the best examples of intergranular corrosion. Other Types of Corrosion. Dissimilar metal corrosion This is one of the types of corrosion where conductor is involved as described above. Such a gap can be formed at the junction of metal . The result is a microstructure that contains extensive amounts of carbide residue, which is immune to IGC. The self-healing property of the passive film make the steel stainless. In austenitic stainless steels, titanium or niobium can react with carbon to form carbides in the heat affected zone (HAZ) causing a specific type of intergranular corrosion known as knife-line attack. Figure 2. The formation of chromium carbides in stainless steel can be prevented by adding the elements titanium (Ti) or Niobium (Nb). Intergranular corrosion is an attack along or near the boundaries of several grains while the rest of the grain remains unaffected. 3. The photos show the typical microstructure of a normalized (unsensitized) type 304 stainless steel and a heavily sensitized steel. Positive identification of this type of corrosion usually Intergranular corrosion (also known as weld decay) affects stainless steel at the structural level and may not show visible signs of damage until corrosion has significantly progressed. This situation can happen in otherwise corrosion-resistant alloys, when the grain boundaries are depleted, known as .mw-parser-output .vanchor>:target~.vanchor-text{background-color:#b1d2ff}grain boundary depletion, of the corrosion-inhibiting elements such as chromium by some mechanism. Localized attack adjacent to grain boundaries with relatively little corrosion of grains. (Related reading: The Role of Chromium in Intergranular Corrosion.) The eight forms are: (1) uniform, or general attack, (2) galvanic, or two-metal corrosion, (3) crevice corrosion, (4) pitting, (5) intergranular corrosion, (6) selective leaching, or parting, (7 . The damage often initiates at end grains encountered in machined edges, holes or grooves and can subsequently progress through an entire section. . Types of Corrosion found in Stainless Steel & Nickel Alloy. Pitting. the one on the right is the "sensitized" structure and is susceptible to intergranular corrosion or intergranular stress corrosion cracking. When looking for stainless steels with excellent intergranular corrosion resistance, consider low-carbon alloys (often designated with an L.)., When choosing low-carbon alloys, options with carbon percentages below 0.03 percent will ensure that insufficient carbon is available for carbide formation., If low-carbon alloys are not appropriate for your intended use, alloys with added titanium or niobium also provide exceptional intergranular corrosion resistance., However, alloys with added titanium or niobium are susceptible to a specialized form of intergranular corrosion called a knife-line attack., This occurs when the carbon interacts with the titanium or niobium instead of the chromium.. After holding senior engineering positions in various manufacturing companies in Canada, he established CorrMet Inspection & Consulting, where he served as Principal. What is Intergranular Corrosion? Also known as intercrystalline corrosion, intergranular corrosion affects grain boundaries. View Full Term. By: Dennis Jayasinghe (Related reading: Why Stainless Steel is Corrosion Resistant.) Intergranular corrosion is another common type of corrosion. In ferritic stainless steels (AISI Type 430, Type 446), the diffusion rate of carbon is so great that the precipitation of chromium carbides cannot be prevented, even with rapid water quenches from high-temperature annealing treatments. Subscribe to our newsletter to get expert advice and top insights on corrosion science, mitigation and prevention. Corrosion, Metallurgy, Failure Analysis and days) Because the chromium is the primary alloying element that makes stainless steel corrosion resistant, the chromium-depleted regions are susceptible to preferential corrosion attack. Intergranular Corrosion Tests. No. Another related kind of intergranular corrosion is termed knifeline attack ( KLA ). | Subject Index | Contact Stress Corrosion Cracking (SCC) . The photos above show the microstructure of a type 304 "Intergranular" or 'intercrystalline" means between Uniform Corrosion Crevice Corrosion. How It Forms Answer The various types of corrosion include Uniform Corrosion, Pitting Corrosion, Crevice Corrosion, Galvanic corrosion, Stress Corrosion Cracking, Intergranular Corrosion, Dealloying, Erosion Corrosion. The first type is common. The chromium depleted zone becomes the Some other forms of corrosion include:. If Intergranular corrosion occurs due to the presence of impurities in the grain boundaries that separate the grain formed during the solidification of the metal alloy. These techniques are expensive and only partially effective since sensitization may occur with time. The cooling rate after this treatment is not important, as the carbon that would otherwise pose risk of formation of chromium carbide is already sequestered as niobium carbide. The cathode to anode ratio is generally greater than one. 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NACE (4) Date. | Principal/Chief Technical Officer, Corr-Met Inspection & Consulting Inc. | Founder & Director, Materials Info Consultancy Private Limited, | Principal Consultant, Bradley Consulting Services, How To Conduct An AC Attenuation Survey on an Underground Pipeline, Anodize This: The Brilliance of Anodizing, How to Prevent Filiform Corrosion Under Coatings, 8 Things to Know and Understand About Iron Corrosion, Top Corrosion Mitigation Technologies to Watch for in 2021, Troubleshooting Cathodic Protection Systems and Function Systems. ), These temperatures are most common during welding, heat treatment, or operation in high-temperature environments., When the metal endures such extreme temperatures, it changes at a structural level., Chromium present in the alloy reacts with carbon to create chromium carbide near the grain boundaries.. However, the material can be tested for resistance to IGC before fabrication of the equipment with specific laboratory methods, such as the Huey test (which uses a nitric solution) or the Strauss test to identify the susceptibility of stainless steel to intergranular corrosion. In the presence of tensile strength, cracking may occur along grain boundaries and this type of corrosion is frequently called "interranular stress corrosion cracking (IGSCC)" or simply "intergranular corrosion cracking". Intergranular attack is caused by the formation of chromium carbides (Fe,Cr)23C6 at grain boundaries, reducing the chromium content and the stability of the passive layer on stainless steels. IGC attacks the material along the grain boundaries (where the grains that make up the metal come together). The loss of cross section thickness and the introduction of cracks can have severe consequences for applications like pressure containment. Medium environment causing intergranular corrosion. Within each grain, the atoms are systematically arranged, forming a three-dimensional lattice. name - "exfoliation". (Source: NASA Corrosion Engineering Laboratory.). Therefore, this type of electrochemical corrosion can cause extensive pitting damage. 5. [4]. To understand intergranular corrosion (IGC), consider that all metals consist of individual grains. Mechanisms of Intergranular Corrosion Impurities at the boundaries cause intergranular corrosion. Fretting Corrosion. Dennis is a registered and active professional engineer in Alberta and Saskatchewan provinces in Canada and a full NACE member. Examination before cleaning is particularly desirable. Titanium, niobium, and their carbides dissolve in steel at very high temperatures. Question 5: What is the tarnishing of silver? Rapid attack at the grain boundaries can result in grains "dropping" or falling out of the metal surface resulting in the disintegration of the steel. Cr-Mn and Cr-Mn-Ni steels are also susceptible to intergranular corrosion following sensitization in the temperature range of 420C850C. Although intergranular corrosion often exists without surface evidence, it can lead to the lifting and flaking of surface metal over time. Also, it has been shown that the zinc content of a brass is higher at the grain boundaries and subject to such corrosion. In aqueous environments, metals may be exposed to not only uniform corrosion, but also to various types of local corrosion including pitting, crevice, intergranular, stress, and galvanic corrosion. Intergranular Corrosion. The alloy disintegrates (grains fall out) and/or loses its strength. (iv) Galvanic Corrosion grains or crystals. IGC is a common form of attack on alloys in the presence of corrosive media that results in the loss of strength and ductility. In nickel alloys and austenitic stainless steels, where chromium is added for corrosion resistance, the mechanism involved is precipitation of chromium carbide at the grain boundaries, resulting in the formation of chromium-depleted zones adjacent to the grain boundaries (this process is called sensitization). Rapid corrosive attack of immediately adjacent grain boundaries with little or no attack of the grains is called Intergranular Corrosion. This phenomenon is analogous to cold rolling process. Figure 1 (b) and 1 (c) show microstructures of stainless steel before and after sensitization, respectively. that progresses preferentially along interdendritic paths (the The ICG localized corrosion at grain boundaries is caused by the anodic dissolution of areas weakened by the alloying elements, second phase precipitation or regions with isolated alloying or impurity elements. However, the rate of chromium diffusion in these alloys is also high. Editorial Review Policy. When an element in a grain that is resistant to corrosion is lost it creates a condition where the spot becomes an anode with reference to the rest . IGC can occur on any equipment fabricated from austenitic stainless steel, nickel-copper alloy, nickel-molybdenum alloy, nickel-chromium alloy, aluminum alloy and zinc alloy in any industry where the right conditions exist, which means that if the material has not undergone proper heat treatment and contains a higher carbon content (C > 0.03%) then it is susceptible. It occurs along elongated grain boundaries in the microstructure. With more than four decades of experience, our sales analysts are ready to assist you with options tailored to the needs of your industry. Intergranular corrosion occurs when certain metals and alloys reach temperatures between 425C and 870C (887F to 1598F. for some alloys and in specific environments. (Cf. SCC This process is known as intergranular corrosion. preferential path for corrosion attack or crack propagation if under tensile Zinc (Zn) of high purity is not prone to IGC. SSC Index. The alloys of aluminum are subject to pitting, intergranular corrosion, and intergranular stress corrosion cracking. However, aluminum as an alloying element or impurities in the zinc alloy could cause an IGC attack. API 571 Damage Mechanisms Affecting Fixed Equipment in the Refining and The remaining part of the exposed surface typically functions as the cathode, and large cathodic areas support the anodic dissolution process. In areas where corrosion is a concern, stainless steel products offer value and protection against these threats. (2 days) Crevice Corrosion. The cracks can follow intergranular or transgranular paths, and there is often a tendency . 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