Stainless steel alloys, corrosion

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). 

Typically, reactors require some type of catalyst. Reactors with catalyst can be of the fixed-bed style for fiuid-bed types. Fixed-bed reactors are the most common. The feed often enters the reactor at an elevated temperature and pressure. The reaction mixtures are often corrosive to carbon steel and require some type of stainless steel alloy or an alloy liner for protection. If the vessel wall is less than 6 mm, the vessel is constmcted of all alloy if alloy is provided. Thicker reactor walls can be fabricated with a stainless overlay over a carbon steel or other lower alloy base steel at less cost than an all-alloy wall constmction. 

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. 

Monel is a type of alloy containing about 70% nickel, with the remainder mainly copper. Except for susceptibility to nitric acid, Monel is generally superior to stainless steel in corrosion resistance. Monel is similar to stainless steel in machinability and is welded like pure nickel. 

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. 

Duplex stainless steel alloys are a mixture of ferritic (400 series) and austenitic (300 series) metals. They provide 1) resistance to stress corrosion and fatigue, 2) pitting resistance, 3) are suitable for a wide temperature range (-50°C to 280°C) and 4) are cost effective. In urea plants, duplex stainless steel is used to construct strippers, decomposers, condensers and pipe lines88. 

In addition to electrodeposition, ionic liquids and DESs can be used in electropolishing, which aims to remove the roughness from metallic surfaces to increase optical reflectivity for high-tech applications. For example, a eutectic mixture of ethylene glycol and choline chloride has been used in the electropolishing of various stainless steel alloys. This method is preferable to current industrial procedures that use a corrosive mixture of phosphoric and sulfuric adds. 

Electrochemical potentlostat measurements have been performed for the corrosion of iron, carbon steel, and stainless steel alloys in supercritical water. The open circuit potential, the exchange or corrosion current density, and the transfer coefficients were determined for pressures and temperatures from ambient to supercritical water conditions. Corrosion current densities increased exponentially with temperature up to the critical point and then decreased with temperature above the critical point. A semi-empirical model is proposed for describing this phenomenon. Although the current density of iron exceeded that of 304 stainless steel by a factor of three at ambient conditions, the two were comparable at supercritical water conditions. The transfer coefficients did not vary with temperature and pressure while the open circuit potential relative to a silver-silver chloride electrode exhibited complicated behavior. 

Passivation consists of exposing a clean metal surface to an oxidizing enviromnent to form an oxide film. This surface is much more corrosion resistant than it would be in an unpassivated state. In materials such as carbon steel, which form weak oxides, passivation can be destroyed rather easily. In oxide-stabilized alloys such as the stainless steels, this corrosion resistance is not easily destroyed. 

W.Y.C. Chen and J.R. Stephens, Anodic Polarization Behavior of Austenitic Stainless Steel Alloys with Lower Chromium Content, Corrosion, Vol 35, 1979, p 443-450... 

Fig. 7.57 Effect of carbon and nickel content on intergranular corrosion penetration rate of 1 8 wt% Cr-base stainless steels. Alloys sensitized for 100 h at 550 °C. Immersion in boiling 65% nitric acid. Pds., periods (48 h) of exposure. Redrawn from Ref 84...

Effect of stress intensity on the growth rate of stress-corrosion cracks in several austenitic stainless steels. Alloy compositions... 

Nickel is used extensively in alloys, notably in stainless steel, other corrosion-resistant alloys such as Monel metal, and coinage metals. Electroplated Ni provides a protective coat for other metals. Nickel has widespread use in batteries recently, this has included the production of environmentally friendly nickel-metal hydride batteries (see Box 9.5) which out-perform NiCd cells (equation 21.5) as rechargeable sources of power in portable appliances. 

Hemihydrate, crystals from water, mp 70-75. Also obtained in anhydr form as monoclinic crystals from water, mp 98°. Obnoxious odor. Strong, corrosive acid. K = 4,6 X 10. Deliquesces rapidly and forms a syrup on short exposure to air. Sparingly sol in ale, ether, benzene. Freely sol in water aq sol ns tend to acquire a yellowish tint. Attacks most base metals except certain stainless steel alloys. 

The most important part of the gauge is the Bourdon tube. Bourdon tubes are made of many materials beryllium copper, phosphor bronze, and various alloys of steel and stainless steel [7]. Beryllium copper is typically used for high pressure applications. Most gauges in air, light oil, or water applications use phosphor bronze. Stainless steel alloys usually add cost to the gauge if specific corrosion resistance is not required. 

The choice of material of construction will influence the extent of possible corrosion and may affect the retention of other deposits, for instance corrosion resistant titanium tubes have been known to foul more rapidly than some stainless steel alloys under similar conditions. It is unlikely in large cooling water systems, e.g. power station condensers, that treatment of the heat exchange surfaces will be considered as a feature for the reduction of fouling, due to the relatively high costs involved including initial cost and maintenance. 

Fig.l Cross-sectional view of intergranular corrosion in a sensitized austenitic stainless steel alloy exposed to boiling H2SO4 + FeCl3. Note that corrosion preferentially occurs along the grain boundaries. (From Corrosion of Stainless Steels A. John Sedriks, copyright John Wiley Sons. Reprinted by permission of John Wiley Sons, Inc.)... 

Flow Lines Pipe and Internal Corrosion Duplex Stainless Steel Alloy... 

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