Titanium nitric acid corrosion

Because of the highly corrosive nature of the nitric acid streams, adipic acid plants are constmcted of stainless steel, or titanium in the more corrosive areas, and thus have high investment costs. 

Titanium is resistant to nitric acid from 65 to 90 wt % and ddute acid below 10 wt %. It is subject to stress—corrosion cracking for concentrations above 90 wt % and, because of the potential for a pyrophoric reaction, is not used in red filming acid service. Tantalum exhibits good corrosion resistance to nitric acid over a wide range of concentrations and temperatures. It is expensive and typically not used in conditions where other materials provide acceptable service. Tantalum is most commonly used in appHcations where the nitric acid is close to or above its normal boiling point. 

For commercially pure titanium, the specific environments to be avoided are pure methanol and red, fuming nitric acid " , although in both environments the presence of 2% of water will inhibit cracking. On the other hand, the presence of either bromine or iodine in methanol aggravates the effect. When it does occur, stress-corrosion cracking of commercially pure titanium is usually intergranular in habit. 

Stress-corrosion cracking (Section 8.10) New metal/environment combinations which produce stress-corrosion cracking are continually being found. Combinations discovered in service in recent years include titanium in red fuming nitric acid carbon steel in liquid anhydrous ammonia and in... 

Under certain conditions it is possible for a weldment to suffer corrosive attack which has the form of a fusion line crack emanating from the toe of the weld this is termed knifeline attack. It is occasionally experienced in welded stabilised steels after exposure to hot strong nitric acids. The niobium-stabilised steels are more resistant than the titanium-stabilised types by virtue of the higher solution temperature of NbC, but the risk may be minimised by limiting the carbon content of a steel to 0-06 Vo maximum (ELC steel). 

Titanium has an unusually high ratio of strength to weight. It is considerably stronger than either aluminum or steel, two metals with which it competes (for special purposes). Its density (4.5 g/cm3) is intermediate between that of Al (2.7 g/cm3) and that of Fe (7.9 g/cm3). Titanium is extremely resistant to corrosion by air, soil, seawater, and even such reactive chemicals as nitric acid and chlorine gas. Like aluminum, it forms a thin, tightly adherent oxide layer that protects the metal from further attack. 

Investigation showed that commercial titanium alloys in contact with acid containing less than 1.34% water and more than 6% of dinitrogen tetraoxide may become sensitive to impact, and react explosively with the acid. Possible causes are discussed [1]. The spongy residue formed by prolonged corrosion of titanium-manganese alloys by red fuming nitric acid will explode on exposure to friction or heat [2],... 

The electrochemical reduction behavior of 1102 was also studied in a nitric acid-hydrazine solution at a titanium electrode because of its resistance to corrosion in nitric acid [59]. It was necessary to pretreat the titanium electrode, to remove surface oxide, through cathodic... 

Although titanium has a large positive E° for oxidation, and T dust will burn in air, the bulk metal is remarkably immune to corrosion because its surface becomes coated with a thin, protective oxide film. Titanium objects are inert to seawater, nitric acid, hot aqueous NaOH, and even to aqueous chlorine gas. Titanium is therefore used in chemical plants, in desalination equipment, and in numerous other industrial processes that demand inert, noncorrosive materials. Because it is nontoxic and inert to body fluids, titanium is even used for manufacturing artificial joints and dental implants. 

Due to its ability to withstand high pressure, its relative low cost, and inertness, stainless steel has become the standard material of columns and other chromatographic components. However, under certain circumstances, stainless steel has been shown to interact with the sample and the mobile phase [39]. The best known example is chloride salt corrosion of stainless steel. Data indicate that nearly all common eluents dissolve iron from stainless steel [39]. It appears that proteins also adsorb to stainless steel [39], The adsorption process is fast, whereas desorption is slow, a result which leads to variable protein recoveries. A number of manufacturers are offering alternatives to stainless components with Teflon -lined columns and Teflon frits. Titanium is being explored as an alternative to stainless steel. A cheaper and simpler procedure is to oxidize the surface of the stainless steel with 6N nitric acid. This procedure should be repeated about every 6 months. 

Titanium is resistant to nitric acid from 65 to 90 wt % and in dilute acid below 10 wt %. It is subject to stress-corrosion cracking above 90 wt %, and it is not used in red fuming acid service because of the potential for a pyrophoric reaction104. 

Majority of titanium alloys are resistant to SCC stress-corrosion cracking has been observed in absolute methanol, red fuming nitric acid, nitrogen tetroxide, liquid, and metals of Cd and Hg and halide media67... 

The corrosion rates of the titanium casing under cathodical conditions were smaller than that of stainless steel in nitric acid ( 0,05 mm Ti/y). This value as well as the corrosion rate of the platinized tantalum anode is within the expected order of magnitude (< 0,005 mm/y). 

Corrosion of Iron, Nickel, Chromium, and Titanium in Sulfuric and Nitric Acids... 

Fig. 5.42 Approximate polarization curves for iron, nickel, chromium, and titanium in 1 N H2S04. Approximate cathodic polarization curves for reduction of nitric acid, dissolved oxygen, and hydrogen ions. An explanation for predicting corrosion behavior based on intersection of anodic and cathodic curves can be found in the text.

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