Corrosion types of metal materials and their testing methods

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Types of corrosion in metallic materials and their testing methods 1. Uniform corrosion Uniform corrosion (also known as general corrosion) refers to the corrosion phenomenon that occurs in a relatively uniform manner across the entire surface of an alloy material. Its morphological characteristic is that, during general corrosion, the thickness of the material gradually decreases, and in some cases corrosion penetrates through it. General corrosion is the basic form of failure that occurs in mechanical equipment during actual use. General corrosion represents the total weight loss of the material. This type of corrosion can be predicted through simple immersion tests, by consulting literature on corrosion, or based on production experience, which facilitates the estimation of equipment lifespan. When selecting corrosion-resistant materials, their overall corrosion resistance is the most fundamental requirement for corrosion resistance. The most commonly used method for uniform corrosion testing is the weight loss method: the specimen is placed in the testing medium, and after a certain period of time, its weight change is measured to determine the corrosion rate. The relevant standard is GB/T10124‑1998 \"Methods for laboratory testing of uniform corrosion by full immersion of metal materials\". 2. Pitting corrosion: Passivated metals are able to resist corrosion because a protective passivation film forms on their surface. However, once this passivation layer is damaged and there are no conditions or capabilities for self-passivation, the metal will corrode. If the corrosion is confined to certain specific areas of the device, resulting in small pits that penetrate deeper into the metal, while most of the metal’s surface remains passive, this phenomenon is known as pitting corrosion. The main test methods for pitting corrosion are electrochemical methods and chemical immersion methods. The electrochemical method mainly measures the breakdown potential of the stainless steel sample. The standard is GB/T17899－1999 \"Method for measuring the pitting potential of stainless steel.\" The chemical immersion method involves using an iron trichloride solution to conduct chemical acceleration tests for pitting. The standard is GB/T17899－1999 \"Test method for pitting corrosion of stainless steel using ferric chloride\". 3. Intergranular corrosion: The grain boundaries are regions where grains with different crystal orientations are misaligned with one another; therefore, they represent favorable areas for the segregation of various solute elements within the metal or for the precipitation of metal compounds such as carbides and σ phases. In certain corrosive media, the grain boundaries may be corroded first. This type of corrosion that occurs first along the boundaries between the grains of a material, resulting in local damage that leads to a loss of cohesion between those grains, is known as intergranular corrosion. Common test methods for intergranular corrosion of stainless steels and alloy steels include: the oxalic acid electrolytic etching method as specified in GB/T4334.1－2000 \"Test Method for Stainless Steels Using 10% Oxalic Acid\", the sulfuric acid–ferrous sulfate method as specified in GB/T4334.2－2000 \"Test Method for Stainless Steels Using Sulfuric Acid and Ferrous Sulfate\", the boiling nitric acid method as specified in GB/T4334.3－2000 \"Test Method for Stainless Steels Using 65% Nitric Acid\", the nitric acid–hydrofluoric acid method as specified in GB/T4334.4－2000 \"Test Method for Stainless Steels Using Nitric Acid and Hydrofluoric Acid\", and the sulfuric acid–copper sulfate method as specified in GB/T4334.5－2000 \"Test Method for Stainless Steels Using Sulfuric Acid and Copper Sulfate\". The selection of the intergranular corrosion testing method is determined based on experience and requirements. The general principle is: for ordinary media, the sulfuric acid-copper sulfate method is used ； The 65% nitric acid method is not used frequently; it is mainly applied in dilute nitric acid media at temperatures ranging from 60°C to the boiling point, as well as in media used for urea synthesis ； Mo-containing stainless steels are generally treated using the nitric acid-hydrofluoric acid method ； The 10% oxalic acid method is mainly used as a screening tool for other methods. 4. Crevice corrosion: Crevice corrosion occurs in dielectric solutions, especially those containing halide ions, within narrow gaps between metal and metal or metal and non-metal surfaces. As the movement of the solution is hindered in these gaps, oxygen is depleted in the solution; as a result, chloride ions migrate from outside the gap into it. The autocatalytic acidification process caused by the hydrolysis of metal chlorides leads to the breakdown of the passivation film, thereby inducing localized corrosion similar to that caused by autocatalytic pitting corrosion. It may damage the integrity and sealing of the mechanical connections, causing serious obstacles or failures in the proper operation of the equipment, and may even lead to destructive accidents. Crevice corrosion usually occurs in gaps where electrolyte solutions (especially those containing halide ions) are trapped, or within shielded surfaces. In general mechanical equipment, gap corrosion tends to occur at flange joints, as well as at locations where there is contact with rivets, bolts, gaskets, washers (especially rubber washers), valve seats, loose surface deposits, and attached marine organisms. It also occurs in the expansion gaps of shell-and-tube heat exchangers. It should be noted that crevice corrosion can occur even in the absence of chloride ions. The commonly used testing methods include chemical immersion with ferric chloride and electrochemical methods. Standard for chemical immersion in ferric chloride: GB/T10127－1998 \"Test method for crevice corrosion of stainless steel using ferric chloride\". 5. Stress corrosion: Under the combined action of stress (tensile stress) and corrosive agents, mechanical equipment components experience brittle cracking that occurs at stresses below the material’s strength limit, leading to the failure of the equipment and components; this phenomenon is known as stress corrosion cracking. Based on the main components of the medium, such as chlorides, hydroxides, nitrates, and oxygenated waters, it is referred to as chloride cracking (chlorine embrittlement or chloride-induced cracking), alkali cracking (alkali embrittlement), nitrate cracking (nitrate embrittlement), and oxygen cracking (oxygen embrittlement), respectively. Stress corrosion cracking is different from cracking caused solely by mechanical stress; it can occur even under extremely low load stresses ； It is also different from cracking caused solely by corrosion; even media with very low corrosivity can cause stress corrosion cracking. Its general corrosion is often mild, and there are no signs of deformation; instead, sudden fracture occurs. Stress corrosion is the most harmful type of destructive corrosion in industrial production. Stress corrosion cracking (SCC) in mechanical equipment and components requires the simultaneous presence of specific conditions related to the material, the environment, and stress. Depending on the method of stress application, stress corrosion testing methods are mainly divided into four categories: (1) Constant deformation method – a certain degree of deformation is applied to the specimen in order to assess its susceptibility to cracking under the test conditions. (2) The constant load method involves applying stress axially to specimens under uniaxial tension, testing them in a corrosive medium, and comparing the fracture times; or, by using the relationship between stress and fracture time, the critical stress for stress corrosion cracking, σscc, can be determined. (3) The slow strain rate method entails using a specially designed stress testing machine with a slow strain rate to stretch the specimens in a corrosive medium until they fracture. The fracture behavior and fracture surface characteristics of the test specimens are analyzed to assess their sensitivity to stress corrosion cracking. (4) Fracture mechanics method: Studies are conducted using wedge-shaped loading specimens; specimens pre-existing with cracks are subjected to various K values to determine the critical value KISCC at which crack propagation stops. The relevant standards include: GB/T4157‑1984 \"Method for constant-load tensile testing of metals for resistance to sulfide stress corrosion cracking\", GB/T17898‑1999 \"Method for stress corrosion testing of stainless steels in boiling magnesium chloride solution\", and GB/T10126‑1988 \"Method for stress corrosion testing of iron-chromium-nickel alloys in high-temperature water\". The main combinations of metal materials and environments that can lead to stress corrosion failure are as follows: (1) Austenitic stainless steels, chlorides, chloride vapors, hydrogen sulfide, alkaline solutions, etc ； (2) Carbon steel and low-alloy steel: media include alkaline solutions, nitrate solutions, anhydrous liquid ammonia, wet hydrogen sulfide, and acetic acid ； (3) Molybdenum-containing austenitic stainless steels: alkaline solutions, chloride aqueous solutions, sulfuric acid + copper sulfate aqueous solutions, etc ； (4) Brass: ammonia and its solutions, ferric chloride, wet sulfur dioxide, etc ； (5) Formic acid or ethanol containing hydrochloric acid titanium, molten sodium chloride ； (6) Aluminum: wet hydrogen sulfide, hydrogen-containing sulfides, seawater.