Stainless steel alloys, corrosion resistance

Washing machines currently on the market are constructed almost exclusively with drums and laundry tubs of corrosion-resistant stainless steel or with an enameled finish that is inert to alkaline wash liquors. Nevertheless, various machine components are made of less detergent resistant metals or alloys. To prevent corrosion of these parts, modern detergents contain corrosion inhibitors in the form of sodium silicate. The colloidal silicate that is present, deposits as a thin, inert layer on metallic surfaces, thereby protecting them from attack by alkali. 

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

The corrosion resistance of steel is significantly improved by alloying with other metals. Highly corrosion-resistant stainless steel is an alloy containing about 15% Cr by mass. 

ACI. Alloy Castings Institute produced a system for corrosion resistant and heat resistant castings. The letter C indicates the corrosion series and the letter H indicates the heat series. For example, CF-8 is a corrosion resistant stainless steel and HK-40 is a heat resistant stainless steel. 

Stabilizing is done on certain corrosion resistant stainless steels and nickel alloys containing titanium, niobium or tantalum to enhance corrosion resistance. They are heated to specific temperatures so that these elements combine preferentially with carbon, preventing any detrimental loss of... 

Aberle D., Agarwal D. (2008), High Performance Corrosion Resistant Stainless Steels and Nickel Alloys for Oil and Gas Apphcations (Paper No. 08085), Houston, TX NACE International. 

Aberle, D., Agarwal, DC. 2008. High performance corrosion resistant stainless steels and nickel alloys for oil and gas applications. NACE—International Corrosion Conference Series, Corrosion 2008 New Orleans, LO March 16-20, 2008 Code 77054, pp. 080851-0808517. 

Some materials are more susceptible to crevice corrosion than others. These materials depend on an oxide film to achieve corrosion resistance. Stainless steel and titanium are prime examples. In addition to improving the design to minimize crevices and to maintain a clean surface on certain materials, alloying of particular materials can be used to improve their resistance to corrosion. 

Pint B A, Peraldi R and Maziasz P J (2004a), The Use of Model Alloys to Develop Corrosion-Resistant Stainless Steels, Mater Sci Forum, 461-464, 815-822. 

Stainless steel alloys show exceUent corrosion resistance to HCl gas up to a temperature of 400°C. However, these are normally not recommended for process equipment owing to stress corrosion cracking during periods of cooling and shut down. The corrosion rate of Monel is similar to that of mild steel. Pure (99.6%) nickel and high nickel alloys such as Inconel 600 can be used for operation at temperatures up to 525°C where the corrosion rate is reported to be about 0.08 cm/yr (see Nickel and nickel alloys). 

Equipment Materials and Abrasion Resistance. Stainless steel, especially Type 316, is the constmction material of choice and can resist a variety of corrosive conditions and temperatures. Carbon steels are occasionally used. Rusting may, however, cause time-consuming maintenance and can damage mating locating surfaces, which increases the vibration and noise level. Titanium, HasteUoy, or high nickel alloys are used in special instances, at a considerable increase in capital cost. 

Ejectors are available in many materials of construction to suit process requirements. If the gases or vapors are not corrosive, the diffuser is usually constructed of cast iron and the steam nozzle of stainless steel. For more corrosive gases and vapors, many combinations of materials such as bronze, various stainless-steel alloys, and other corrosion-resistant metals, carbon, and glass can be used. 

Note that sensitization will not result in weld decay in all environments. Stainless steels may be used in environments that do not require the full corrosion resistance of the alloy. In these cases, weld decay may not occur even though sensitization has taken place. 

In this chapter we look first at an important class of alloys designed to resist corrosion the stainless steels. We then examine a more complicated problem that of protecting the most advanced gas turbine blades from gas attack. The basic principle applicable to both cases is to coat the steel or the blade with a stable ceramic usually Cr203 or AI2O3. But the ways this is done differ widely. The most successful are those which produce a ceramic film which heals itself if damaged - as we shall now describe. 

This unit can he fahricated of a wide range of ferrous, stainless steels, and nonferrous corrosion resistant metals and alloys. 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

Stainless steel alloys are in wide use in many technological applications where corrosion resistance is a concern. Because the corrosion properties of these steels are very important, they have been widely studied by many methods, including considerable use of surface analytical tools. The nature of the protective films formed on these alloys in different environments is of great interest. 

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