Identify alloys and alloying elements that resist caustic environments better th

Question

Identify alloys and alloying elements that resist caustic environments better than iron and simple steels.

(b) Draw a simple stress corrosion map for carbon steel in caustic.

(c)Describe the regions of potential where stainless steels may suffer SCC in hot caustic.

(d) Describe the main types of stress corrosion that occur in hot water, with and without dissolved oxygen and/or hydrogen

(e)Explain the action of noble metals on stainless steel in hot water containing both hydrogen and oxygen

. (f) Explain the mechanisms of stress corrosion in hot water

Explanation / Answer

By adding chromium, nickel, molybednum and carbon in the steel we can improve the corrosion resistance of steel.

Caustic cracking of stainless steel occurs over wide range of caustic concentrations. The lower limit of cracking appears to be in the range of few percent by weight of caustic as NaOH. In kraft cooking liquors the concentration of hydroxides is of the order of 3to 5 percent which is sufficient to induce cracking in all conditions.

Stainless steel appear to be resistant to cracking in kraft cooking liquors at impregnation zone temperatures, although cracking can occur when the temperatures are higher, when the caustic concentration are increased or when stainless steel microstructure is sensitized by improper heat treatment.

Hydrogen dissolves in all metals to a moderate extent. Hydrogen tends to attract towards regions of high triaxial tensile stress where the metal structure is dilated. The dissolved hydrogen then assit in the fracture of metal possibly by making cleavage easier. This effects leads to embrittle ment of the metal,

Chloride stree corrosion cracking can occur in hot water jacket around chocolate pipes in the food industry. In this situation we can not easily change the materail or temperature and its imposible to eliminate stress associated with welding and forming of stainless steel.

Dissolved oxygen cause crevice corrosion.Stainless steel require oxygen to make passive layer on the surface. In very tight crevices it is not possible for oxygen to gain access to the stainless steel thereby caising it to vunerable to attack.

Galvanic corrosion can occur when it comes in contact with noble metals in hot water. Stainless steel act as nobler metal in their passive state. If the surface of the less noble metal is small relative to the nobler metal, the corrosion rate become very high.If carbon steel bolts are used to fasten stainless steel sheets which can lead to severe corrosion on the bolts. Defects in coating and paint on a less noble metal can result in small anodic area which can lead to strong corrosion. Oxidation reaction occur at the metal surface and reduction treaction occur at the stainless steel surface.

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