Chemical resistance to corrosives

Chemically Resistant Fibers. Fibers with exceUent chemical resistance to corrosive and/or chemical warfare agents or extreme pH conditions (eg, very acidic or very alkaline) were initially used for protective clothing. However, appHcations for filtration of gases and Hquids in numerous industrial faciHties are now the more important. For example, PPS is suitable for use in filter fabrics for coal-fired boilers because of its outstanding chemical and heat resistance to acidic flue gases and its exceUent durabUity under these end use conditions. Many high tenacity fibers are also chemically inert or relatively unaffected under a variety of conditions. Aramids, gel spun polyethylene, polypropylene, fluorocarbon, and carbon fibers meet these criteria and have been used or are being considered for appHcations where chemical resistance is important. 

Successful operation of the gaseous diffusion process requires a special, fine-pored diffusion barrier, mechanically rehable and chemically resistant to corrosive attack by the process gas. For an effective separating barrier, the diameter of the pores must approach the range of the mean free path of the gas molecules, and in order to keep the total barrier area required as small as possible, the number of pores per unit area must be large. Seals are needed on the compressors to prevent both the escape of process gas and the inflow of harm fill impurities. Some of the problems of cascade operation are discussed in Reference 16. 

A type of E-glass fibre with enhanced chemical resistance to corrosion. 

Chemical resistance good chemical resistance to corrosive environments ... 

Heating, ventilation, and air conditioning (HVAC) applications rely on SPS s hydrolytic and heat resistance for moisture drip pans that are close to heating elements, and chemical resistance to corrosive flue gases generated by combustion. Excellent dimensional stability ensures consistent manufacturing within the tight tolerances required for precision components. 

Stainless steel. A group of Cr or Cr —Ni steels showing an unusually high resistance to corrosion by the atmosphere and many chemical reagents. In order to achieve this resistance to corrosion > 12% Cr is necessary. 

Gold [7440-57-5] Au, is presumably the first metal known and used by humans. It occurs ia nature as a highly pure metal and is treasured because of its color, its extraordinary ductility, and its resistance to corrosion. Early uses ia medicine and dentistry date to the ancient Chinese and Egyptians. In the Middle Ages the demand for gold led to the iatense, unsuccesshil efforts of alchemists to convert base metals iato gold. These pursuits became the basis for chemical science. The search for gold has been an important factor ia world exploration and the development of world trade. 

The principal advantage of plastic dmms and liners is their resistance to corrosion. This aspect of their performance requires the lading to be investigated in terms of capacity for chemical attack on the dmm. Stress-cracking tests should be performed in all instances where the compatibiUty of jading and dmm material has not been estabUshed (6). 

In dry air at room temperature this reaction is self-limiting, producing a highly impervious film of oxide ca 5 nm in thickness. The film provides both stabihty at ambient temperature and resistance to corrosion by seawater and other aqueous and chemical solutions. Thicker oxide films are formed at elevated temperatures and other conditions of exposure. Molten aluminum is also protected by an oxide film and oxidation of the Hquid proceeds very slowly in the absence of agitation. 

This is used widely for chemical and process plant applications where its resistance to corrosion and high thermal conductivity are desirable characteristics. The strength of aluminum can be increased by cold working, as shown in Table 3.29. 

All raw materials and chemicals are checked for meeting the specifications set in the manufacturing procedures by standardized methods. PSS uses extremely high quality steel (V4A quality), which has proven to be especially resistant to corrosive compounds and/or conditions. 

The most important nitride of Si is Si3N4 this is formed by direct reaction of the elements above 1300 or more economically by heating Si02 and coke in a stream of N2/H2 at 1500". The compound is of considerable interest as an engineering material since it is almost completely inert chemically, and retains its strength, shape and resistance to corrosion and wear even above 1000°. ° Its great hardness (Mohs 9), high... 

The majority of metals and alloys available depend for their resistance to corrosion on the properties of an oxide film or corrosion product which is formed initially by the corrosion process. In many cases the protectiveness of the oxide film is determined by its stability in aqueous solutions in a specific pH range, either chemically dissolving to form aquocations at lower pH values or complex anions (aluminate, ferroate, plumbate, zincate, etc.) at higher pH values (Fig. 2.22). An important property of the chemical is therefore the pH value that it develops when dissolved in water. For many materials and many chemicals this is the overriding factor and in many cases... 

In the polyacrylic synthetic fibre industry, carbonitrided molybdenum guides have been used in place of chromium plated steel because of their resistance to corrosion and erosion. Chemicals that attack molybdenum are listed in Table 5.9. 

It is in its behaviour to caustic alkalis that zirconium shows itself to be superior to those other elements of Groups IV and V whose resistance to corrosion results primarily from an ability to form surface films. Thus, in contrast to tantalum, niobium and titanium, zirconium is virtually completely resistant to concentrated caustic solutions at high temperatures, and it is only slightly attacked in fused alkalis. Resistance to liquid sodium is good. Zirconium is thus an excellent material of construction for sections of chemical plant demanding alternate contact with hot strong acids and hot strong alkalis—a unique and valuable attribute. 

The outstanding characteristics of the noble metals are their exceptional resistance to corrosive attack by a wide range of liquid and gaseous substances, and their stability at high temperatures under conditions where base metals would be rapidly oxidised. This resistance to chemical and oxidative attack arises principally from the Inherently high thermodynamic stability of the noble metals, but in aqueous media under oxidising or anodic conditions a very thin film of adsorbed oxygen or oxide may be formed which can contribute to their corrosion resistance. An exception to this rule, however, is the passivation of silver and silver alloys in hydrochloric or hydrobromic acids by the formation of relatively thick halide films. 

All the six platinum-group metals are highly resistant to corrosion by most acids, alkalis, and other chemicals. Their high nobility is the main factor determining their chemical resistance, and the formation of. complex ions in solution is principally responsible for their dissolution under certain conditions. 

Resistance to corrosion of electroless nickel, both as-deposited and, in most cases, after heating to 750°C, is listed by Metzger for about 80 chemicals and other products. Resistance was generally satisfactory, with attack at a rate below 13 /im/year. The only substances causing faster attack were acetic acid, ammonium hydroxide or phosphate, aerated ammonium sulphate, benzyl chloride, boric acid, fluorophosphoric acid, hydrochloric acid, aerated lactic acid, aerated lemon juice, sodium cyanide and sulphuric acid. 

Impervious graphites, that is electro-graphites with appropriate resin impregnation are used in cascade-, shell- apd tube-type coolers, condensers, pre-heaters etc. in a wide variety of chemical plants. Similar resistance to corrosion applies to glassy carbon vessels and pyrolytic carbons and graphites. The corrosion resistance to principal chemical agents is given in Table 18.2. 

One of the most important properties of commercial glasses is their great resistance to corrosion any chemical laboratory apparatus, any window or windscreen provides an excellent illustration. Windows remain virtually unchanged for centuries, resisting the influences of atmosphere and radiation. A vast range of products may be safely stored in glass for decades at ordinary temperatures, and the fact that gleiss can be used with alkaline, neutral and acid environments allows the same equipment to be used for a variety of processes. 

Owing to their almost complete resistance to corrosion, stoneware and porcelain occupy a unique position as materials in the chemical industry. Stoneware bodies can be compounded from many different raw materials to give the properties required and to permit the manufacture of articles up to a considerable size. Porcelain, on the other hand, is not generally used for very large articles, but owing to its completely vitreous nature it normally shows a much greater resistance to corrosion than does stoneware. 

---