Contamination, by corrosion

Tanks for the transport of chemicals cannot usually be cathodically protected because of the danger of impurities contaminating the cargo. Particular emphasis is placed on the quality of the coating to avoid contamination by corrosion products. 

The metal lost from the inside of pumps, reaction vessels, pipework, etc. usually contaminates the product. The implications of this depend upon the product. Ppb levels of iron can discolor white plastics, though at this level the effect is purely cosmetic. Ppm levels of iron and other metals affect the taste of beer. Products sold to compositional requirements (such as reagent-grade acids) can be spoiled by metal pick-up. Pharmaceutical products for human use are often white tablets or powders and are easily discolored by slight contamination by corrosion products. 

In most cases, condensate does not require treatment prior to reuse. Makeup water is added directly to the condensate to form boiler feedwater. In some cases, however, especially where steam is used in industrial processes, the steam condensate is contaminated by corrosion products or by the inleakage of cooling water or substances used in the process. Hence steps must be taken to reduce corrosion or to remove the undesirable substances before the condensate is recycled to the boiler as feedwater. 

Dead spots and crevices - where equipment parts are not continuously wetted by oxygen-containing liquids - are prone to severe corrosion. Therefore, fabrication of this equipment should be done to avoid such vulnerable spots. An effort should also be made to reduce the potential for process contamination by corrosive agents such as sulfur (through oil in liquid NH3), H2S (along with C02) and chlorides (from cooling water)88. 

As to the membrane conductivity, only small losses of protonic conductivity, of the order of 5-10% after 4000 h, have been observed in well-humidified cells during PEFC life tests according to measurements of cell impedance at 5 kHz [42]. The deionized water employed in the humidification scheme [42] had very low levels of metal ions (e.g., Fe " / +, Ca + or Mg +). Such multivalent ions could exchange irreversibly with protons in the PFSA membrane, causing a drop in membrane conductivity. Deionizing the water used for PEFC humidification is therefore required, and appropriate plumbing should also be used in the humidification loop to avoid generation of ionic contaminants by corrosion processes. 

This step in the process presents very severe corrosion conditions since die apparatus parts are subjected to the action of a hot, acid solution saturated with oxygen. Such materials of construction may only be used as will insure a satisfactory equipment life and a product free from contamination by corrosion products. 

The treatment of cooling water is a subject for specialists. Requirements are affected by the quality of the water supply the extent of contamination by corrosion products, process leakage, and the use of the water in direct-contact applications and the nature and quantity of solids and gases scrubbed from the air in the cooling tower. There are extreme variations in some of these factors, and each plant will need its own treatment system and program. 

Another example is a process in which stainless steels are used to prevent product contamination by corrosion products. In such cases, even carbon steel may have the necessary corrosion resistance from an engineering standpoint, but the corrosion products formed by the small amount of corrosion of carbon steel are not acceptable, and for this reason a stainless steel is used, e.g., to prevent discoloration of polymers on certain textile fiber plants. Finally, sensitization may not be a problem when stainless steel is used for appearance or ease of maintenance. 

If the atmosphere is clean but not dry and the humidity approaches 100%, a scattered pattern of corrosion spots eventually appears, but considerable areas of unaffected surface remain for a very long time. If, however, the surface becomes contaminated by corrosive dust or cathodic particles, the whole surface rapidly becomes covered with a grayish layer of corrosion product (Tawil, 1987). A clean, unprotected magnesium alloy surface exposed to indoor or outdoor atmospheres free from salt spray develops a gray Him that protects the metal from corrosion (Froats et al., 1987). The rates of corrosion and resistance to corrosion of magnesium alloys vary depending on alloy composition (Loose, 1946) as discussed previously. 

Various methods have been developed for measuring many of the factors that influence atmospheric corrosion. The quantity and composition of pollutants in the atmosphere, the amount collected on surfaces under a variety of conditions, and the variation of these with time have been determined. Temperature, RH, wind direction and velocity, solar radiation, and amount of rainfall are easily recorded. Not so easily determined are dwelling time of wetness (TOW), and the surface contamination by corrosive agents such as sulfur dioxide and chlorides. However, methods for these determinations have been developed and are in use at various test stations. By monitoring these factors and relating them to corrosion rates, a better understanding of atmospheric corrosion can be obtained. 

Corrosion is a threat to the environment. For instance, water can become contaminated by corrosion products and unsuitable for consumption. Corrosion prevention is integral to stop contamination of air, rater and soil. The American Water Works Association needs US 325 billion in the next twenty years to upgrade the water distribution system. 

The system fluid may be a further source of external contamination. New oil as refined and blended is clean (contaminant free), but if stored in a bulk tank or in drums is subject to contamination from metal and rubber particles in the filling lines, and possibly metal flakes or scale from the container itself. Water condensation within tanks further contributes to contamination by corrosion products. Typical contaminant size in fluids is 3 to 10 jm, but much larger particles may be present in bad cases. 

The problems inherent to these two processes are not only the production of corrosive salts, but also the possibiUty of product contamination by 2-chloroethylamine [689-98-5] as starting material or intermediate. This substance can initiate polymerisation of ethyleneimine with the elimination of HCl. 

Techniques for handling sodium in commercial-scale appHcations have improved (5,23,98,101,102). Contamination by sodium oxide is kept at a minimum by completely welded constmction and inert gas-pressured transfers. Residual oxide is removed by cold traps or micrometallic filters. Special mechanical pumps or leak-free electromagnetic pumps and meters work well with clean Hquid sodium. Corrosion of stainless or carbon steel equipment is minimi2ed by keeping the oxide content low. The 8-h TWA PEL and ceiling TLV for sodium or sodium oxide or hydroxide smoke exposure is 2 mg/m. There is no defined AID for pure sodium, as even the smallest quantity ingested could potentially cause fatal injury. 

Condensate Polishing. Ion exchange can be used to purify or poHsh returned condensate, removing corrosion products that could cause harmful deposits in boilers. Typically, the contaminants in the condensate system are particulate iron and copper. Low levels of other contaminants may enter the system through condenser and pump seal leaks or carryover of boiler water into the steam. Condensate poHshers filter out the particulates and remove soluble contaminants by ion exchange. 

The use of neutralising amines in conjunction with an oxygen scavenger—metal passivator improves corrosion control in two ways. First, because any acidic species present is neutralized and pH is increased, the condensate becomes less corrosive. Second, most oxygen scavenger—passivators react more rapidly at the mildly alkaline conditions maintained by the amine than at lower pH levels. For these reasons, this combination treatment is gaining wide acceptance, particularly for the treatment of condensate systems that are contaminated by oxygen. 

Another difficulty sometimes encountered in laboratory tests is that contamination of the testing solution by corrosion products may change its corrosive nature to an appreciable extent. 

Sodium and potassium are restricted because they react with sulfur at elevated temperatures to corrode metals by hot corrosion or sulfurization. The hot-corrision mechanism is not fully understood however, it can be discussed in general terms. It is believed that the deposition of alkali sulfates (Na2S04) on the blade reduces the protective oxide layer. Corrosion results from the continual forming and removing of the oxide layer. Also, oxidation of the blades occurs when liquid vanadium is deposited on the blade. Fortunately, lead is not encountered very often. Its presence is primarily from contamination by leaded fuel or as a result of some refinery practice. Presently, there is no fuel treatment to counteract the presence of lead. 

Many systems are idle for long periods after operating at high temperatures. This permits moisture to condense in the system, resulting in rust formation. Certain corrosion-and rust-preventive additives are added to hydraulic liquids. Some of these additives are effective only for a limited period. Therefore, the best procedure is to use the liquid specified for the system for the time specified by the system manufacturer and to protect the liquid and the system as much as possible from contamination by foreign matter, from abnormal temperatures, and from misuse. 

The failure of plant by corrosion can be gradual or catastrophic. Gradual failure has few implications for safety providing it is monitored. Direct corrosion-monitoring techniques are described in Section 53.8. Indirectly, the correct interpretation of records relating to metal contamination of products or the loss of efficiency of heat exchangers, etc. can provide useful information. 

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

The shape of a vessel determines how well it drains (Figure 53.7). If the outlet is not at the very lowest point process liquid may be left inside. This will concentrate by evaporation unless cleaned out, and it will probably become more corrosive. This also applies to horizontal pipe runs and steam or cooling coils attached to vessels. Steam heating coils that do not drain adequately collect condensate. This is very often contaminated by chloride ions, which are soon concentrated to high enough levels (10-100 ppm) to pose serious pitting and stress corrosion cracking risks for 300-series austenitic stainless steel vessels and steam coils. 

Excessive lubrication Fatigued race or rolling elements Fretting wear Contamination by Inadequate abrasive or lubrication corrosive materials Grease churning due to too soft consistency... 

It is common experience that corrosive soils tend to be the heavy clays, especially if they have been subjected to working by, for example, heavy earth-moving machinery. Lighter soils are usually only corrosive if they have been contaminated by industrial debris, especially ashes, ferrogenous slags and carbonaceous material such as cinders. 

The corrosion of tin by nitric acid and its inhibition by n-alkylamines has been reportedThe action of perchloric acid on tin has been studied " and sulphuric acid corrosion inhibition by aniline, pyridine and their derivatives as well as sulphones, sulphoxides and sulphides described. Attack of tin by oxalic, citric and tartaric acids was found to be under the anodic control of the Sn salts in solution in oxygen free conditions . In a study of tin contaminated by up to 1200 ppm Sb, it was demonstrated that the modified surface chemistry catalysed the hydrogen evolution reaction in deaerated citric acid solution. 

Higher acidity caused greater corrosion but contamination by sulphur dioxide or carbon dioxide inhibited attack. By contrast, chloride ions were found to have a mild aggressive effect. 

Sulphur attack on nickel-chromium alloys and nickel-chromium-iron alloys can arise from contamination by deposits resulting from the combustion of solid fuels, notably high-sulphur coals and peat. This type of corrosion, which has been observed on components of aircraft, marine and industrial gas turbines and air heaters, has been associated with the presence of metal-sulphate and particularly sodium sulphate arising directly from the fuel or perhaps by reaction between sodium chloride from the environment with sulphur in the fuel. Since such fuels are burned with an excess of air, corrosion occurs under conditions that are nominally oxidising although the deposits themselves may produce locally reducing conditions. 

When mature concrete is contaminated by chloride, e.g. by contact with deicing salts, the cement chemistry is more complex, and less chloride is taken up by the cement hydrate minerals and a larger proportion is free in the pore solutions and can therefore pose a greater hazard. When embedded steel corrodes, the production of a more voluminous corrosion product pushes the concrete from the steel with resultant cracking and spalling of the concrete. 

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