Sulfuric acid passive high-concentration

Lead s durability (its chemical inertness) and malleability make it useful in the construction industry. The inertness of lead under normal conditions can be traced to the passivation of its surface by oxides, chlorides, and sulfates. Passivated lead containers can be used for transporting hot concentrated sulfuric acid but not nitric acid, because lead nitrate is soluble. Another important property of lead is its high density, which makes it useful as a radiation shield because its numerous electrons absorb high-energy radiation. The main use of lead today is for the electrodes of rechargeable storage batteries (see Box 12.1). 

Hot, concentrated nitric acid will dissolve all common metals with the exception of aluminum and chromium, which are passive to the reagent as a result of surface oxide formation. Hot nitric acid also readily oxidizes many organic substances. Hot, concentrated sulfuric acid can be used to decompose and dissolve many substances in part because of its high boiling point (340°C), and it is particularly useful for the dehydration and oxidation of organic samples. Most metals and alloys are also attacked by this hot acid. 

From a chemical point of view, plutonium is a reactive metal, and hence its powder is highly pyrophoric. Moreover, plutonium readily dissolves in dilute sulfuric (H SO ), concentrated hydrochloric (HCl), hydroiodic (HI), trichloroacetic, orthophosporic (H PO ), and perchloric (HCIOJ acids. However, it dissolves slowly in hydrofluoric acid (HF) and resists nitric acid (HNO3) and passivates into concentrated sulfuric acid. 

Nitric acid has a high oxidizing capacity and stainless steels will remain passive in nitric acid solutions. Although sulfuric acid is corrosive to stainless steels, it may be innocuous in low concentrations and in the presence of aeration. Similar performance can be expected in phosphoric acid and in many of the organic acids. 

The chloride pitting resistance of this alloy is similar to that of type 316 stainless steel and superior to that of types 430 and 439L. Like all ferritic stainless steels, t)/pe 444 relies on a passive film to resist corrosion, but exhibits rather high corrosion rates when activated. This characteristic explains the abrupt transition in corrosion rates that occur at particular acid concentrations. For example, it is resistant to very dilute solutions of sulfuric acid at boiling temperature, but corrodes rapidly at higher concentrations. 

Anodic protection s is a modern electrochemical technique for protecting metallic equipment used in the chemical-process industry against corrosion and handling highly corrosive chemicals (e.g., concentrated sulfuric and orthophosphoric acids). The technique consists in impressing a very low anodic current (i.e., usually 10 irA.m" ) on a piece of metallic equipment (e.g., tanks, thermowells, columns) to protect against corrosion. This anodic polarization puts the electrochemical potential of the metal in the passivity region of its Pourbaix... 

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