Nitric acid concentrated, environment-alloy

Behavior in chemical environments can be briefly stated in terms of resistance of copper and its alloys to acids and bases. Acids such as acetic, phosphoric, dilute sulfuric and hydrochloric acids can be handled, providing there are no oxidizing agents present. Nitric and concentrated sulphuric acids dissolve copper and its alloys and cannot be tolerated. The copper and its alloys, as is the case with any system, should be tested in acid systems or any other environment of interest closely simulating the industrial plant operating conditions before finalizing the operation of the plant. 

Type 304 stainless steel is basically an alloy of 18 to 19 wt% Cr and 8 to 10 wt% Ni. Its corrosion behavior in sulfuric acid is sensitive to both alloy composition and the sulfuric acid environment. Variables with respect to alloy composition include whether the Cr and Ni concentrations are high or low within the allowed range and the concentrations of residual elements such as sulfur, phosphorus, copper, and molybdenum. Thermal and mechanical treatments are also variables but are not considered in the following. Important variables with respect to the sulfuric acid environment include degree of aeration and agitation (velocity effect) and small concentrations of species such as nitric acid, cupric ions, and ferric ions. The net influence of these variables is to find corrosion rates varying from <25 pm/year (1 mpy) to >2500 pm/year (100 mpy) (Ref 3 9). 

Crevice corrosion occurs mainly (but not exclusively) on passive materials. The most important problem is the crevice corrosion of stainless steels, nickel-base alloys, aluminum alloys, and titanium alloys in aerated chloride environments, particularly in sea or brackish water, but also in environments found in chemical, food, and oil industries. Other cases of crevice corrosion are also known such as the so-called corrosion by differential aeration of carbon steels, which does not require the presence of chloride in the environment. Also mentioned in the literature is the crevice corrosion of steels in concentrated nitric acid and inhibited cooling water and of titanium alloys in hot sulfixric environments. 

The acidity or alkalinity of the environment significantly affects the corrosion behavior of aluminum alloys. At lower and higher pH, aluminum is more likely to corrode but by no means always does so. For example, aluminum is quite resistant to concentrated nitric acid. When aluminum is exposed to alkaline conditions, corrosion may occur, and when the oxide film is perforated locally, accelerated attack occurs because aluminum is attacked more rapidly than its oxide under alkaline conditions. The result is pitting. In acidic conditions, the oxide is more rapidly attacked than aluminum, and more general attack should result. 

Its chromium content gives the alloy resistance to sulfur compounds and various oxidizing environments. The chromimn content of the alloy makes it superior to commercially pure nickel under oxidizing conditions. In strong oxidizing solutions hke hot, concentrated nitric acid, 600 has poor resistance. Alloy 600 is relatively unattacked by the majority of neutral and alkaline salt solutions and is used in some caustic environments. The alloy resists steam and mixtures of steam, air, and carbon dioxide. 

The primary advantage of anodic protection is its applicability for extremely corrosive environments. It is most extensively applied to protect equipment used to produce, store, and handle sulfuric acid. It is also used in chemical and nuclear industries, during the production of fertilizers, and for the protection of heat exchangers and tankers with hot concentrated adds sulfuric, nitric, phosphoric, ammonium nitrate, and so on, involving components usually made of carbon steels, alloy steels, stainless steels, titanium, nickel and its alloys, and so on. 

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