Resistance to High Acidity

Some bacteria are able to withstand high acidity in their surroundings, and for some pathogens this is critical for their survival. Some details of this are given in Chapter 7, Part 1, and it is sufficient here to merely note the existence of mechanisms whereby the decarboxylation of aminoacids serves to remove protons from the medium. 

Ackerley DF, CF Gonzalez, CH Park, R Blake, M Keyhan, A Matin (2004) Chromate-reducing properties of soluble flavoproteins from Pseudomonas putida and Escherichia coli. Appl Environ Microbiol 70 873-882. 

Adaikkalam V, S Swarup (2002) Molecular characterization of an operon, cueAR, encoding a putative Pl-type ATPase and a MerR-type regulatory protein involved in copper homeostasis in Pseudomonas putida. Microbiology (UK) 148 2857-2867. 

Alvarez AH, R Moreno-Sanchez, C Cervantes (1999) Chromate efflux by means of the ChrA chromate resistance protein from Pseudomonas aeruginosa. J Bacterial 181 7398-7400. 

Avazeri C, R Turner, J Pommier, J Weiner, GG Giordano, A Vermeglio (1997) Tellurite resistance activity of nitrate reductase is responsible for the basal resistance of Escherichia coli to tellurite. Microbiology (UK) 143 1181-1189. 

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Zinc dibenzyldithiocarbamate (formula 19.1), which is soluble in CCI4 and CHCI3, is less selective than Pb(DDTC)2 but more resistant to highly acidic media [20]. With this reagent, copper can be extracted from 1-2 M HCl or H2SO4. Lead dibenzyldithiocarbamate has been used similarly [21]. 

High chromium (20—27%) SS provides better corrosion resistance to nitric acid at elevated temperatures than low carbon SS. It has been used to... 

Glass offers good resistance to strong acid at high temperatures. However, it is subject to thermal shock and a gradual loss in integrity as materials such as iron and siUca are leached out into the acid. Nonmetallic materials such as PTFE, PVDC, PVDF, and furan can be used for nitric acid to a limited degree, but are mainly restricted to weak acid service at ambient to moderate temperatures. 

Zirconium resists attack by nitric acid at concentrations up to 70 wt % and up to 250°C. Above concentrations of 70 wt %, zirconium is susceptible to stress-corrosion cracking in welds and points of high sustained tensile stress (29). Otherwise, zirconium is resistant to nitric acid concentrations of 70—98 wt % up to the boiling point. 

Platinum and Platinum Alloys. Platinum has excellent resistance to strong acids and, at elevated temperatures, to oxidation. Under reducing conditions at high temperatures it must be protected from low-fusing elements or their oxides. Easily reduced metals at high temperatures may form low-fusing alloys with platinum. 

They show good to excellent resistance to highly aromatic solvents, polar solvents, water and salt solutions, aqueous acids, dilute alkaline solutions, oxidative environments, amines, and methyl alcohol. Care must be taken in choice of proper gum and compound. Hexafluoropropylene-containing polymers are not recommended for use in contact with ammonia, strong caustic (50% sodium hydroxide above 70°C), and certain polar solvents such as methyl ethyl ketone and low molecular weight esters. However, perfluoroelastomers can withstand these fluids. Propylene-containing fluorocarbon polymers can tolerate strong caustic. 

Polypropylene has a chemical resistance about the same as that of polyethylene, but it can be used at 120°C (250°F). Polycarbonate is a relatively high-temperature plastic. It can be used up to 150°C (300°F). Resistance to mineral acids is good. Strong alkalies slowly decompose it, but mild alkalies do not. It is partially soluble in aromatic solvents and soluble in chlorinated hydrocarbons. Polyphenylene oxide has good resistance to ahphatic solvents, acids, and bases but poor resistance to esters, ketones, and aromatic or chlorinated solvents. 

Polysulfone can be used to I70°C (340°F) it is highly resistant to mineral acid, alkali, and salt solutions as well as to detergents, oils, and alcohols. It is attacked by such organic solvents as ketones, chlorinated hydrocarbons, and aromatic hydrocarbons. 

Fluorinated Excellent resistance to high temperature, oxidizing acids, and oxidation good resistance to fuels containing up to 30% aromatics... 

Polysulfide Good resistance to aromatic solvents unusually high impermeability to gases poor compression set and poor resistance to oxidizing acids... 

Aluminum is resistant to nitric acid at concentrations above 80%. At 50% nitric acid concentration at room temperature, corrosion rates are as high as 0.08 in. (0.20 cm) per year. 

Aluminum and aluminum-clad steels am highly resistant to naphthenic acids under most conditions. Aluminum-coated steels give good service until coatings fail at coating imperfections, cracks, welds, or other voids. In general, the use of aluminum and aluminized... 

The commercial polymers are generally resistant to aqueous acids and alkalis although they are attacked by concentrated sulphuric acid. As might be expected of a highly polar polymer it is not dissolved by aliphatic hydrocarbons but solvents include dimethyl formamide and dimethyl acetamide. 

The properties of the polyurethane moulding compositions are also very similar to nylon 66. The greatest difference in properties is in water absorption, the 6,4-polyurethane absorbing only about of that of nylon 66 under comparable conditions. This results in better dimensional stability and a good retention of electrical insulation properties in conditions of high humidity. Resistance to sulphuric acid is somewhat bettter than with nylon 66 but both types of polymer are dissolved by phenols and formic acid. 

Combination metallic and nonmetallic cloths consist of metallic wires and weak cloth or asbestos threads. There are some difficulties in weaving when attempting to maintain uniformity between wires and the cloth, and considerable dissatisfaction has been experienced with such construction. While cotton weaves well with the asbestos, the cotton fibers destroy the fabric s resistance to heat and corrosion. Its use is, therefore, quite limited, despite its resistance to high temperatures, acids and mildew. 

Chlorinated rubber paints and vinyl paints have excellent resistance to high concentrations of acids and alkalis at temperatures up to 80°C. High-build chlorinated rubber paints, which give a thickness of 0.12 mm per coat are commonly used for process plant equipment. 

The super austenitics with high contents of nickel and molybdenum with some copper have much enhanced resistance to sulphuric acid solutions. 

AUS104 AUS204 - Good resistance to high temperature oxidation good corrosion resistance in sulphuric acid Pumps and valves... 

The data available suggest that high-chromium irons have no useful resistance to sulphuric acid of more than 10% concentration at any temperature. At temperatures above 20°C corrosion rates in excess of 1 27 mm/y are probable even for acid of less than 10% concentration. The addition of 2% molybdenum appears to produce an appreciable increase in the resistance to this acid at very low and very high concentrations (Fig. 3.54). 

High nickel austenitics, with 25-35% Ni, 20-22% Cr and up to 6% Mo, with good resistance to reducing acids. 

High silicon austenitics, containing 4-6% Si, with good resistance to highly oxidising nitric and sulphuric acids. 

As a future alternative to glassed steel there is ceramics-coated steel which is resistant to abrasion, corrosion and high temperatures. The base metal is coated with silicon nitride formed in situ. Silicon nitride has resistance to both acid and alkali and it is durable at temperatures up to 1 000°C, suggesting a promising future coating in aggressive operating environments. 

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