Corrosive agent

STM has been used to study adsorption on surfaces as it relates to corrosion phenomena [6, 7]. Sulfiir is a well known corrosion agent and is often found in air (SO2, FI2S) and in aqueous solution as dissolved anions ( HSO7) or dissolved gas (FI2S). By studying the interaction of sulfur with surfaces, insights can be gained into... 

A steel surface can be wetted and corroded by permeation of corrosive agents (see Section 5.2.2) and by cathodic disbonding (see Section 5.2.1.5). In all cases the corrosion rates are negligibly small (see Table 5-2). In the case of cathodic... 

C. Permeability of the matrix material to corrosive agent such as water... 

Korrosions-bestilndigkeit, /. corrosion resistance. -bildner, m. corrosive agent. 

Reactors should not dissolve in the reaction medium. Judging by spectro-graphic analysis of spent catalysts, some attack of the reactor is more common than is generally supposed. It may be a cause of catalyst failure. Reactors are commonly made of type 316 stainless steel, but other alloys may provide better resistance to spedhc corrosive agents. 

As mentioned earlier, microorganisms can attack drilling fluid additives and introduce corrosive agents to the system. Therefore, it is very important to monitor their activity and detect any source of problem as early as possible. API RP 38 is probably the most widely used testing procedure in the industry [201]. The methods that can be used to monitor the microbial activity can include the following [201,208] ... 

Water quality is important, not only from an environmental point of view but also in relation to the type of packing to be specified. Analysis of the circulating water is simple to obtain, but it is very seldom offered to the cooling tower designer. The quality, or lack of it, will determine the type of pack to be used, the selection of structural materials and whether the tower should be induced or forced draft, counterflow or crossflow. Water treatment, in the shape of chemicals to control pH and to act as counter-corrosion agents or as biocides, all has a bearing on tower selection. 

Corrosion is generally taken to be the waste of a metal by the action of corrosive agents. However, a wider definition is the degradation of a material through contact with its environment. Thus, corrosion can include non-metallic materials such as concrete and plastics and mechanisms such as cracking in addition to wastage (i.e. loss of material). This chapter is primarily concerned with metallic corrosion, through a variety of mechanisms. 

The external environment experienced by plant can be more corrosive than the internal process stream. Any con-stmction material must be chosen to withstand or be able to be protected from external corrosive agents. 

Because of their generally poor resistance to solvents, acids, alkalis and other corrosive agents, paints are not normally used to protect plant internals handling anything... 

In the gas plant, corrosive agents (H,S, HCN, and NHj) are r concentrated at high-pressure points. Water is usually injected into the first and second-stage compressor discharges. The water contacts the hoi gas and scrubs these agents. There are two common injection methods forward cascading and reverse cascading. 

Sea water is the only electrolyte containing a relatively high concentration of salts that occurs commonly in nature, covering as it does over two-thirds of the earth s surface. It is both the most familiar and one of the most severe of natural corrosive agents. 

To prevent underground corrosion, lead is frequently protected with coatings of tar, bitumen, resin, etc., which are only effective if they completely insulate the metal from corrosive agents and stray currents. No coating is fully effective, but some give good protection ". The most successful method used is cathodic protection which for impressed currents, if correctly applied, can protect indefinitely (see Chapter 10). It is effective at a potential of E° = —0-8 V or about 0-1 V more negative than... 

In this section, chemical resistance will be divided into three parts, viz. acid, alkali (including detergents) and water (including atmosphere). Normally an enamel is formulated to withstand one of the corrosive agents more specifically than another, although vitreous enamel as a general finish has good all round resistance, with a few exceptions such as hydrofluoric acid and fused or hot concentrated solutions of caustic soda or potash. 

Environments are either gases or liquids, and inhibition of the former is discussed in Section 17.1. In some situations it would appear that corrosion is due to the presence of a solid phase, e.g. when a metal is in contact with concrete, coal slurries, etc. but in fact the corrosive agent is the liquid phase that is always present. Inhibition of liquid systems is largely concerned with water and aqueous solutions, but this is not always so since inhibitors may be added to other liquids to prevent or reduce their corrosive effects — although even in these situations corrosion is often due to the presence of small quantities of an aggressive aqueous phase, e.g. in lubricating oils and hydraulic fluids (see Section 2.11). 

Terminology The International Standards Organization has recently defined a corrosion inhibitor as a chemical substance which decreases the corrosion rate when present in the corrosion system at a suitable concentration, without significantly changing the concentration of any other corrosive agent. This last point is significant since it excludes chemicals employed for deaeration or pH control from the definition of a corrosion inhibitor. On the other hand, it should be noted that the inhibitor is .. . present in the corrosion system. . . , and thus arsenic when added to brasses to prevent dezin-cihcation may be classified as an inhibitor. 

General corrosion properties The glass surface may react with a corrosive agent in one or a combination of the following ways ... 

For the most common series of corrosive agents, water, steam, acids, alkalis and salts, the hydrolytic processes peculiar to each determine the mechanism of attack. Thus, under the right circumstances, hydrolytic attack on the bridging oxygens can occur in the following way ... 

Attack by alkali solution, hydrofluoric acid and phosphoric acid A common feature of these corrosive agents is their ability to disrupt the network. Equation 18.1 shows the nature of the attack in alkaline solution where unlimited numbers of OH ions are available. This process is not encumbered by the formation of porous layers and the amount of leached matter is linearly dependent on time. Consequently the extent of attack by strong alkali is usually far greater than either acid or water attack. 

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