In our surroundings, there have six main distinction kinds of corrosion: uniform corrosion, galvanic corrosion, pitting corrosion, crevice corrosion, intergranular corrosion, and dealloying. To begin with, uniform corrosion is the commonly expected mode of corrosion that the metal from the surface are regular removed. For this corrosion, the exposed metal surface probably has the same rate corrosion proceeds in the corrosive environment. Uniform corrosion can be prevented by a multitude of methods. Initially, the humankind can use thicker materials to improve corrosion allowance. A second way is using paints or metallic coating to restrain corrosion, in particular, plating, galvanizing, and anodizing are extremely terrific examples. Furthermore, using corrosion inhibitors or modifying the environment is one of the best ways to deter the corrosion. Moreover, cathodic and anodic protections are usually applied in prevention uniform corrosion. Then, galvanic corrosion is defined that the two different alloys are coupled presence in the corrosive electrolyte, one of them is prior corroded in order to protect the other from corrosion. To be more specific, any two dissimilar alloys have unlike corrosion potentials (Ecorr), on the one hand, more positive alloy will be preferentially corroded. On the other hand, the more noble alloy will be protected from corrosion. In particular, galvanic corrosion is most intense at interface and decreases with distance.
A variety of ways can be used to prevent galvanic corrosion: firstly, possible close together metals or alloys are choosing in the galvanic series to restrain corrosion; secondly, a diminutive anode and big cathode are avoiding in unfavorable area; thirdly, the dissimilar metals can be insulated; furthermore, coating is reasonable applying in the prevention galvanic corrosion. Pitting corrosion is the localized attack of metal surface that bring about cavities in the metal. The environment (may contain chloride) and material (defects or may contain MnS) factors can caused pitting corrosion. A number of possible countermeasures are using in prevention pitting corrosion: first of all, known resistance to the environment materials can be selected to prevent corrosion; in addition, according to control chloride concentration restrain pitting corrosion; after that, increasing resistance to pitting corrosion is using higher alloys(ASTM G48). Crevice corrosion has some similar with pitting corrosion that the localized attack of metal surface or right away adjacent to the crevice between two joining surfaces. The difference concentration of some chemical constituents (usually oxygen) establishes an electrochemical concentration cell: outside of the crevice (the cathode) is the oxygen content; inside the crevice (the anode) is chlorides concentration. There are many approaches to prevent crevice corrosion: initially, similar with the pitting corrosion, increasing resistance to crevice corrosion is using higher alloys (ASTM G48); moreover, solid, non-absorbent gaskets (such as Teflon) can be used to prevented crevice corrosion; ultimately, using welding or soldering technique is one of the effective ways to deter the crevice corrosion. Intergranular corrosion is in the presence of tensile stress situation, along grain boundaries may be occurred cracking. Intergranular means between grains and crystals, according to this name, this is a form of corrosion attack along grain boundaries. In order to prevent this type of corrosion, post â€"weld heat treatment and low carbon grade of stainless steels can be used to prevent intergranular corrosion. Dealloying is a solid solution alloy that selects corrosion of one or more than one component; in other words, dealloying means prating or selective attack. Case in point, dezincification and graphitic corrosion are the common examples. A number of methods can use to prevent corrosion: first of all, in order to dealloying is choosing more resistant metals or alloys; subsequently, the environment can be control to minimize the selective leaching.
(2) Explain the different types of stainless steel
Stainless steel is the alloy steel (iron-based and chromium), which is using commonly in our daily life. Stainless steel corrosion resistance depends on the contained alloy element in the steel. These alloy elements are including nickel, molybdenum, titanium, nitrogen and so on, adding some alloy elements aim to meet the various stainless steel organizations and performance requirements. However, the main stainless steel alloy element is chromium, only when the chromium content reaches a certain value (at least 10.5%) and the content of carbon normally less than 1.2%, then steel have corrosion resistance. Adding distinction levels of varied alloys can create different kinds of stainless steel. Basically, stainless steel can divide into five major types: austenitic stainless steel, martensitic stainless steel, ferritic stainless steel, duplex stainless steel, and precipitation hardening stainless steel. These stainless steel have own individual composition and properties as follows.
Figure stress corrosion cracking in ASS
Austenitic stainless steel has another name the 300 series, which is the most prevalent and comprehensively used steel in our contemporary society. It contains at least 18% chromium and approximately 8% nickel, also includes a little bit other element such as nitrogen, manganese and nickel. It has terrific comprehensive performance and highly resistant to a variety of corrosion. Likewise, austenitic stainless steels have fantastic hygiene properties and adapt to a multitude of work surroundings (both low and high temperatures). However, stress corrosion cracking can be easily assessed to standard austenitic steels and occurs in the room temperature not only in hot condition (figure); hence resistance to stress corrosion cracking is increased by higher nickel austenitic steels. In addition, austenitic stainless steel is almost non- magnetic. To sum up, because austenitic stainless steel owns those characteristics, it is commonly used in producing large members of goods, for instance kitchen equipment, chemical equipment and pumps. Martensitic stainless steel has the highest content of carbon (about 1%) in the five types of stainless steels, at the meantime includes nearly 18% chromium. Additional materials can be found in the martensitic stainless steel, for example nickel and molybdenum. Due to this type of steel has higher carbon content therefore its own high strength; at the same time it is weakness of resistance corrosion. Moreover, austenitic stainless steel has some magnetism and can be hardened by application in high temperature. Actually, this type of steel was first using commercial area, and nowadays it was used to be produce cutlery and surgical equipment. After that, ferritic stainless steel owns at least 27% chromium and carbon’s content about 10.5%, together with martensitic stainless steel are called as the 400 series. Due to it has higher content of chromium, thus this type of steel has superb quality to prevent corrosion. Similarly, it cans safety use in seawater because the higher content of chromium and adds some molybdenum. However, it is distinction with other types of steel, to be more specific, ferritic stainless steel cannot be hardened by intense heating and it is less ductile than austenitic stainless steel or martensitic stainless steel. Compared with austenitic stainless steel, this steel is usually using their resistance to stress corrosion cracking; likewise, it is magnetic. In conclusion, ferritic stainless steels are generally making molds and car trim. Then, duplex stainless steel has contains approximately 50% austenitic and 50% ferritic steel; hence, it owns these two types of stainless steel advantages. This means that duplex stainless steel it is not only higher resistance to stress corrosion cracking, but also it owns higher strength and some magnetism. Furthermore, the molybdenum in the duplex stainless steel can boost pitting resistance. ‘Superduplex’ is one of the best qualities of stainless steel and also has the highest prevention corrosion character. Precipitation hardening stainless steel has tremendous mechanical strength, but it is inferior of corrosion resistance and easily lead to cracking. And the application restricts mild environments.
