Corrosion. Also

Other methods of metal powder manufacture are also employed for specific metals. Selective corrosion of carbide-rich grain boundaries in stainless steel, a process called intergranular corrosion, also yields a powder. 

Corrosion also occurs as a result of the conjoint action of physical processes and chemical or electrochemical reactions (1 3). The specific manifestation of corrosion is deterrnined by the physical processes involved. Environmentally induced cracking (EIC) is the failure of a metal in a corrosive environment and under a mechanical stress. The observed cracking and subsequent failure would not occur from either the mechanical stress or the corrosive environment alone. Specific chemical agents cause particular metals to undergo EIC, and mechanical failure occurs below the normal strength (5aeld stress) of the metal. Examples are the failure of brasses in ammonia environments and stainless steels in chloride or caustic environments. 

Finally, pitting may be viewed as a special form of concentration cell corrosion. Most alloys that are susceptible to crevice corrosion also pit. However, many metals may pit but not show crevice attack. Further, although sharing many common features with concentration cell corrosion, pitting is sufficiently different to warrant a separate categorization. 

Fine chemicals, foods, and pharmaceuticals The immunity of tantalum to corrosion also ensures product purity and undesired side reactions in the processing of fine chemicals, foods, and pharmaceuticals. 

The thermodynamic phase stability diagrams appear to be preferred by corrosion scientists and technologists for the evaluation of gas-metal systems where the chemical composition of the gaseous phase consisting of a single gas or mixture of gases has a critical influence on the formation of surface reaction products which, in turn, may either stifle or accelerate the rate of corrosion. Also, they are used to analyse or predict the reason for the sequence of formation of the phases in a multi-layered surface reaction product on a metal or alloy. 

Stress-corrosion cracking occurs in certain aluminium alloys which have been developed for medium and high strength by employing variations in composition, cold work and heat treatment The main alloys are based upon Al-Mg, Al-Mg and Al-Cu, but stress corrosion also occurs in Al-Ag, Al-Cu-Mg, Al-Mg-Si, Al-Zn and Al-Cu-Mg-Zn alloys. It has... 

Interest in this metal comes from its remarkable inertness to atmospheric corrosion. Also, it is very hard and thus it forms an ideal protective coating. On the basis of its E° (1.18 volts higher than hydrogen) we expect chromium to be quite reactive in fact, it is vigorously reactive with... 

Corrosion also occurs where only one metal is involved. Here variations in oxidation potential caused by surface chemistry differences (such as irregularities in the metal s crystalline structure or stresses caused during the finishing stages of manufacture) create microanodes and microcathodes. 

Fouling, like deposition and corrosion, also takes many forms but generally involves the physical adherence to waterside surfaces of materials such as oils, process contaminants, corrosion products, and settled boiler sludges. 

Some measure of control over corrosion also is obtained by limiting the salinity in the boiler (primarily the ions of sodium, chloride, and sulfate). These ions all increase the conductivity of boiler water electrolyte and thus enhance corrosion reaction rates. Also, chloride and sulfate ions affect the passivation process. 

Sodium The FW sodium (Na) content is clearly a factor in the formation of sodium hydroxide in BW and an excess may promote various forms of caustic-induced corrosion. Also, high sodium levels may lead to the depassivation of steel surfaces caused by high pH generation, which reduces the corrosion resistance of boiler steel. 

NOTE Under these low hardness but high MU demand circumstances, other risk factors also exist, especially in smaller boilers. Typically, the high MU water demand reduces the FW temperature, which permits the oxygen level to rise and thus requires additional sulfite scavenger to counter the increased risk of corrosion. Also, the requirement for BD is increased as TDS and SS levels also rise, so fuel and other operating costs increase accordingly. 

The direct reaction of ferric oxide particles with steel heat transfer surfaces (corrosion) also is possible, producing a thick, coarse, and porous deposit of magnetite. 

NOTE Caustic gouging corrosion also may occur in LP boilers because the deliberate provision of a caustic reserve is normally a basic waterside chemistry requirement. 

Oxygen in a large operating boiler may corrode steam-water separators and boiler surface components such as the top drum (especially at the waterline) and tubes. Oxygen corrosion also may occur in superheater and reheater tubes, especially in places where moisture can collect, such as in bends and sagging tubes. 

Oxygen corrosion also may occur if and when the following conditions below ... 

All common amines present at least a moderate degree of skin and eye irritation. Most are corrosive. Also, CHA is toxic and is described as an extremely hazardous substance. 

Lay-ups provide an opportunity to remove these deposits indeed, they must be removed because they attract moisture and promote fireside corrosion. Also, when the temperature of the boiler falls below the dewpoint, very corrosive sulfuric acid is formed (the pH of the deposit frequently falls below 3.0). 

Nitric acid is corrosive. Also, note that the CUSO4 solution contains sulfuric acid and hydrochloric acid. Wash any spills on your skin with plenty of cold water. Inform your teacher immediately. 

One of the best known examples of electrochemical corrosion is atmospheric rusting. For this to occur, a certain critical relative humidity of between 60-80% or higher (depending upon whether salts are present) is required. At such a relative humidity, every object is covered with a coherent film of water which serves as an electrolyte. Electrochemical corrosion also occurs when an iron object is partly or completely immersed in water. 

Corrosion-inhibiting properties of fuel stabilizers can be a secondary effect of the dispersing action of a fuel stabilizer. By functioning as a dispersant, sludge and water are held in suspension and prevented from initiating metal surface corrosion. Also, some stabilizer dispersants can form a thin film on the metal surfaces of fuel system components. This film-forming property enables the stabilizer-dispersant to function in corrosion control. A typical oil-soluble dispersant compound is shown in FIGURE 6-3. 

Coffee andteaaTe decaffeinated using cacbon dioxide in a fourth phase of matter known as a supercritical fluid. This phase behaves like a gaseous liquid, which is attained by adding lots of pressure and heat. Supercritical carbon dioxide is relatively easy to produce. To get water to form a supercritical fluid, however, requires pressures in excess of 217 atmospheres and a temperature of 374°C. Supercritical water is very corrosive. Also, so much oxygen can dissolve in supercritical water that flames can burn within this medium, which is ideal for the destruction of toxic wastes. 

Equipment at high or low temperatures is insulated to conserve energy, to keep process conditions from fluctuating with ambient conditions, and to protect personnel who have occasion to approach the equipment. A measure of protection of the equipment metal against atmospheric corrosion also may be a benefit. Application of insulation is a skilled trade. Its cost runs to 8-9% of purchased equipment cost. 

An electrochemical mechanism for corrosion also explains nicely why automobiles rust more rapidly in parts of the country where road salt is used to melt snow and ice. Dissolved salts in the water droplet greatly increase the conductivity of the electrolyte, thus accelerating the pace of corrosion. 

Procedure Setup the chlorine generating apparatus as illustrated below. Note your chlorine generator does not have to be as fancy as the apparatus illustrated below, but your system should be similar in design. You can use latex tubing, PVC pipes and other plastic devices as well, steel pipes can be used, but other metals should be avoided due to corrosion. Also, observe the chlorine generator utilizing an electrochemical method. The electrochemical method is sufficient to produce laboratory quantities of gas (see the following illustration). 

As you know from Chapter 4, solid solutions of metals are called alloys. Adding even small quantities of another element to a metal changes the properties of the metal. Technological advances throughout history have been linked closely to the discovery of new alloys. For example, bronze is an alloy of copper and tin. Bronze contains only about 10% tin, but it is much stronger than copper and more resistant to corrosion. Also, bronze can be melted in an ordinary fire so that castings can be made, as shown in Figure 8.3. 

Spent acid is handled very carefully. It is very corrosive. Also, its H2S04 and hydrocarbons may continue to react during storage and transport (Lang, 1998). 

Resistant to high pitting corrosion. Also available as 316L for welded construction. 

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