Corrosion Properties of 1Cr17Ni6Mn5N (AISI201) Austenitic Stainless Steel
Introduction
The 1Cr17Ni5Mn6N (AISI201) austenitic stainless steel is a relatively new member of the chromium-nickel family of stainless steels. It belongs to the group of ferrite-austenite duplex stainless steels that contain between 9.2-13.0 wt%. of chromium, 8-13 wt%. of nickel and 0.15-0.45 wt%. of nitrogen. This type of stainless steel offers a combination of galvanic corrosion resistance in addition to improved resistance to various types of localized corrosion and abrasion. Moreover, its mechanical properties offer enhanced strength and ductility. As a result, the 1Cr17Ni5Mn6N (AISI201) stainless steel has enjoyed considerable application in various industrial sectors.
This paper attempts to evaluate the corrosion resistance of the 1Cr17Ni5Mn6N (AISI201) stainless steel in a number of corrosive environments. The information presented is based on tests conducted by a number of research institutions and the authors’ own studies.
Similarly, knowledge on its corrosion mechanisms, inhibitors and surface treatments will be discussed. In addition, information on the corrosion failures commonly associated with this alloy will also be presented. Furthermore, the paper will discuss the current trends in the use of 1Cr17Ni5Mn6N (AISI201) stainless steel and potential areas of improvement.
Corrosion Resistance
Resistance to stress corrosion cracking.
1Cr17Ni5Mn6N (AISI201) stainless steel generally exhibits better resistance to stress corrosion cracking than other stainless steel alloys in chloride-containing environments, due to its relatively low levels of alloys, compared to other stainless steel alloys. The alloy does however, exhibit susceptibility to intergranular attack in cases where excessive temperature has been applied or in cases of stress. Microstructural examination suggests that the intergranular attack is caused by the formation of carbides at the grain boundaries. To avoid this issue, it is recommended that the alloy be heated at slower rates, and the nominal nitrogen content should be kept as high as possible.
Resistance to pitting and crevice corrosion.
1Cr17Ni5Mn6N (AISI201) stainless steel exhibits better resistance to pitting and crevice corrosion than other stainless steel alloys in chloride-containing environments. The resistance is mainly attributed to its higher chromium, nickel and nitrogen contents, which are all known for forming protective oxide films that serve to protect the metal from corrosion. The alloy, however, is still susceptible to pitting and crevice corrosion under certain conditions. In general, localized corrosion of this alloy is promoted by high solution chloride levels, stagnant conditions, improper pH and inadequate surfaces treatments. Proper passivation techniques can also be employed to improve pitting and crevice corrosion resistance.
General corrosion resistance.
The general corrosion resistance of 1Cr17Ni5Mn6N (AISI201) stainless steel is excellent, due to the presence of a thin, protective oxide film that forms on the metal’s surface. The protective film is formed by the chromium, nickel and nitrogen alloys, which react with oxygen and water to form an adherent and passive metal oxide film. This film generates a strong repulsion force against any corrosive attack. The alloy is generally resistant to most acids and bases, except for nitric and sulfuric acid, which can cause slight to moderate pitting.
Inhibition and Protection
There are many methods available for protecting 1Cr17Ni5Mn6N (AISI201) stainless steel from corrosion. In mild environments, one of the simplest methods is to employ an organic coating, such as paint or varnish, in order to create an impermeable barrier layer. In other cases, a metallic coating layer can be used to form a barrier layer or an applied protective film on the metal’s surface. In more severe environments, where corrosion-resistant alloys cannot be used, corrosion-resistant materials, such as thermal sprays and cladding materials, can also be employed.
In addition to protective coatings, corrosion inhibitors can also be used to protect 1Cr17Ni5Mn6N (AISI201) stainless steel from corrosion. Corrosion inhibitors are chemicals that act to reduce the rate of corrosion by forming a coating over the metal’s surface and blocking oxidation reactions that would normally occur. Commonly used inhibitors for stainless steel include nitrates, phosphates and chromates. Nondestructive testing (NDT) methods can also be used for the purpose of detecting thinning of the metal or other signs of corrosion on exposed parts of the stainless steel.
Surface Treatments
The surface treatment of 1Cr17Ni5Mn6N (AISI201) stainless steel is important in order to ensure maximum corrosion resistance and durability. The surface of the alloy should be free from dirt, grease and other organic and inorganic compounds prior to treatment. This can be achieved through the use of mechanical cleaning methods, such as blasting and grinding. Once the surface has been cleaned, a passivation step should be performed to remove iron-containing compounds from the surface. This will ensure that the protective oxide film forms properly on the metal’s surface.
Failure Analysis
The use of 1Cr17Ni5Mn6N (AISI201) stainless steel can lead to corrosion failure under certain conditions. Poor corrosion resistance can occur due to incorrect heat treatment, incorrect machining, welding defects or the presence of excess alloying elements or contaminants. In some cases, these issues can be prevented or minimized by proper selection and control of the manufacturing process parameters and materials. In addition, proper pre- and post-treatment of the material can help to improve corrosion resistance and performance.
Conclusion
The 1Cr17Ni5Mn6N (AISI201) austenitic stainless steel offers a combination of excellent resistance to chloride-induced stress corrosion cracking, pitting, crevice corrosion and general corrosion, making it a suitable choice for use in a variety of corrosive environments. In order to maximize the corrosion resistance of this alloy, proper surface treatment, coating and corrosion inhibitors should be employed where necessary. Additionally, the application of NDT is also recommended to detect any signs of corrosion. Through correct selection, preparation and application of 1Cr17Ni5Mn6N (AISI201) stainless steel, its excellent corrosion resistance can be leveraged for optimum performance.
