Aggressivity of water

Oxygen in water is important when evaluating aggressivity of water towards metals (so-called oxygen corrosion), and it is an important index for checking the operation of biological wastewater treatment plants. 

Hydrogen carbonates also favourably affect the taste of water. To ensure non-aggressivity of water, minimum values of HCO3 have been defined for water transported by piping. It is recommended that the concentration of hydrogen carbonates in drinking water should be higher than 0.8 mmol 1 . 

Many authors discuss the theoretical and experimental solution of the calcium-carbonate equilibrium [14]. The methods are based on various simplifying assumptions and they solve calcium-carbonate equilibrium and thus also aggressivity of water with greater or lesser success. In the majority of cases the aggressivity of water can be evaluated only semi-quantitatively. 

The evaluation of aggressivity according to the Langelier index of saturation is not ideal. The calculated pHg values are usually higher than the actual ones. The aggressivity of waters with low mineralization is normally overestimated, and that of the waters with high mineralization is underestimated. 

The pH value considerably influences the course of chemical and biochemical processes in waters. It causes the differentiation of the speciflc forms of occurrence of some elements, it is one of the aspects for the evaluation of the aggressivity of water and it influences the eflUciency of many technological processes employed for the treatment and puriflcation of waters. 

Usually concrete withstands normal potable water because the free Ume reacts with CO2 forming CaC03, which is not readily soluble and forms a protective skin on the concrete surface. Only in soft water with a high concentration of free CO2 is this protective layer dissolved by the formation of soluble Ca(HC03)2. A special case is the attack of concrete by very soft waters (e. g. condensation) that dissolve the free Hme (solubility 1.7 g/1). When free Hme is dissolved or leached out, the other constituents of the cement paste are attacked and the resistance of the concrete diminishes. The aggressiveness of waters to concrete can be classified, e. g. according to EN 206 [4], where pH, free CO2 content, ammonium, magnesium and sulfate ions are taken as criteria (Table 3.3). 

Plasticizer can also be extracted from PVC by a range of solvents including water. The aggressiveness of a particular solvent depends on its molecular size and its compatibiUty with both the plasticizer and PVC. Water extracts plasticizer very slowly, oils are slightly mote aggressive, and low molecular weight solvents are the most aggressive. 

Turbulence and high fluid velocities resulting from normal pump operation accelerated metal loss by abrading the soft, graphitically corroded surface (erosion-corrosion). The relatively rapid failure of this impeller is due to the erosive effects of the high-velocity, turbulent water coupled with the aggressiveness of the water. Erosion was aided in this case by solids suspended in the water. 

The horizontal surfaces should be coated because there is residual water in the ballast and there are water-oil mixtures in the crude oil tanks when ships travel empty and these can cause severe corrosion attack. In the lower part of the tank, up to about 1.5 m from the base, a combination of coating and cathodic protection with special anodes is chosen. Basically the anodes could take over the exclusive protection in this area, but with empty ballast tanks containing residual water or empty crude oil tanks with aggressive oil-water mixtures containing sulfur compounds, they do not prevent corrosion. 

More recently, attempts have been made to correlate mathematically the chemical composition of natural waters and their aggressivity to iron by direct measurements on corrosion coupons or pipe samples removed from distribution systemsThis work has been of limited success, either producing a mathematical best fit only for the particular data set examined or very general trends. The particular interest to the water supply industry of the corrosivity of natural waters to cast iron has led to the development of a simple corrosion rig for the direct measurement of corrosion ratesThe results obtained using this rig has suggested an aggressivity classification of waters by source type i.e. 

The above catalogue of difficulties, in relating the aggressivity of natural waters to their chemical composition, arises precisely because of the low corrosion rates that are usually found with most metals. Under such circumstances, water composition is only one of many factors that determine the rate of attack. The other factors include flow regime, temperature and the conditions under which the initial corrosion product is laid down. The best summary of the behaviour of metals commonly used in natural waters is still that produced by Campbell for the Society of Water Treatment and Examination... 

Those waters in which the carbon dioxide content is in excess of that required as bicarbonate ion to balance the bases present are among the most aggressive of the fresh waters. Hard waters usually, though not invariably, deposit a carbonate scale and are generally not appreciably corrosive to cast iron, corrosion rates of less than 0-02 mm/y being frequently encountered. Water-softening processes do not increase the corrosivity of the water provided that the process does not result in the development of an excess of dissolved carbon dioxide. 

Establishing electrolyte resistivity To enable a satisfactory cathodic-protection scheme to be designed, it is necessary to determine the resistivity of the electrolyte (soil or water). This information is necessary to enable the current output of anodes to be determined together with their position and power source voltage, and it also provides an indication of the aggressiveness of the environment in general the lower the resistivity the more aggressive the environment. 

Open recirculating systems These are more amenable to inhibition since it is possible to maintain a closer control on water composition. Corrosion inhibition in these systems is closely allied to a number of other problems that have to be considered in the application of water treatment. Most of these arise from the use of cooling towers, ponds, etc. in which the water is subject to constant evaporation and contamination leading to accumulation of dirt, insoluble matter, aggressive ions and bacterial growths, and to variations in pH. A successful water treatment must therefore take all these factors into account and inhibition will often be accompanied by scale prevention and bactericidal treatments. 

Make environment less aggressive by removing constituents that facilitate corrosion decrease temperature, decrease velocity where possible prevent access of water and moisture. 

Methods used to control presumptive corrosion include deaeration and dehydration. Carbon dioxide and hydrogen sulfide are the main corrosives in pipelines for natural gas, but they are only aggressive in the presence of water. Therefore sweetening and drying the gas are useful to prevent corrosion. In oil pipelines, water emulsified in crude oil can cause corrosion problems [251]. Emulsified crude oil in separated produced water is also an environmental and disposal problem. 

The effectiveness of chlorine reduces markedly as the pH of the system water rises above 7 and at pH s above 8 is largely ineffective as a disinfectant. High levels of chlorine are also known to be aggressive to many of the materials of construction of water services. 

Foam—stable aggression of small bubbles of lower density than oil or water, which shows tenacious qualities in covering and clinging to vertical or horizontal surfaces. Foam flows freely over a burning liquid surface, forming a tough, air-excluding continuous blanket to seal volatile combustible vapors from access to air. 

Adhesives and sealers can be an important part of a total corrosion protection system. Structural bonding procedures and adhesives for aluminum, polymer composites, and titanium are well established in the aerospace industry. Structural bonding of steel is gaining increasing prominence in the appliance and automotive industries. The durability of adhesive bonds has been discussed by a number of authors (see, e.g., 85). The effects of aggressive environments on adhesive bonds are of particular concern. Minford ( ) has presented a comparative evaluation of aluminum joints in salt water exposure Smith ( ) has discussed steel-epoxy bond endurance under hydrothermal stress Drain et al. (8 ) and Dodiuk et al. (8 ) have presented results on the effects of water on performance of various adhesive/substrate combinations. In this volume, the durability of adhesive bonds in the presence of water and in corrosive environments is discussed by Matienzo et al., Gosselin, and Holubka et al. The effects of aggressive environments on adhesively bonded steel structures have a number of features in common with their effects on coated steel, but the mechanical requirements placed on adhesive bonds add an additional level of complication. 

The relative aggressiveness of the environments proved to be consistent for all substrates, with the room temperature control the least hostile (virtually no loss of adhesion), and the cycle tests the most aggressive (up to 100% loss of adhesion within 60 days). Humidity cabinet exposure and 60°C water immersion yielded very similar values. As a result, for reasons of clarity, only water immersion data is actually presented here. Joint strength data obtained from either the Ford APG or Fisher Body Cycle Tests were identical, and were therefore also represented by one set of data points. The relative aggressiveness of the host environments toward... 

The conventional technology of water electrolysis makes use of alkaline solutions [7]. In particular, about 30% KOH is used at about 80 °C. The use of KOH, although more expensive than NaOH, is dictated by two reasons (1) KOH is more conductive (about 1.3 times) than NaOH and (2) KOH is chemically less aggressive than NaOH. A 30% concentration is used because the conductivity exhibits a maximum there. 

Dew or condensation of humidity is considered an important cause of the corrosion of metals. Its formation depends on the relative humidity and on the changes of temperature. Dew does not wash the metallic surface so the concentration of pollutants is relatively high and could be more aggressive than rain. Rain gives rise to the formation of a thick layer of water and also adds corrosive agents such as H+ and S042" however it can wash away the contaminants as well. It will depend on the intensity and duration of the rainfall. 

Table 3 illustrates the composition of four mine waters. It will be noted that sulphide ore mines tend to generate more aggressive mine water than coal mines, and that recently flooded mines tend to generate poorer water qualities than long-... 

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