Specific Corrosion Protection Measures

Another significant market imperfection is the lack of technology transfer that leads to the lack of awareness about corrosion and the available methods to combat corrosion. There are many instances where unnecessary costs are incurred because of incorrect specification of materials or where inadequate corrosion protection measures have been applied to proprietary equipment. The information that is required to avoid the corrosion problem was available and could have been used to control the corrosion problem. 

The American Water Works Association (AWWA) has developed a numerical soil corrosivity scale that is applicable to cast iron alloys. A severity ranking is generated by assigning points for different variables, presented in Table 2.28. When the total points of a soil in the AWWA scale are 10 (or higher), corrosion protective measures (such as cathodic protection) have been recommended for cast iron alloys. It should be appreciated that this rating scale remains a relatively simplistic, subjective procedure for specific alloys. Therefore, it should be viewed as a broad indicator and should not be expected to accurately predict specific cases of corrosion damage. 

Calcium nitrite should not be viewed as an alternative to the design specification for durable concrete nor as a means to "improve" poor quality concrete. Calcium nitrite corrosion inhibitor in combination with good quality concrete is a viable means to achieving long-term durability. Calcium nitrite can be used combination with other corrosion protection measures. 

Corrosion Resistance. The environment to which a plated part is to be exposed should be a part of any definition of corrosion resistance. Problems arise in testing a part in its intended environment in part because of the long time period required. In many plating processes, corrosion resistance is direcdy proportional to the plate thickness, so a specification on plate thickness is a much faster method of indirectiy measuring corrosion resistance. In specifying a corrosion resistance requirement for the production of plated goods, accelerated tests are used especially if plate thicknesses caimot be related to corrosion protection. 

Next to sales contract specifications, coiTosion protection ranks highest among the reasons for the removal of acid gases. The partial pressure ol the acid gases may be used as a measure to determine whether treatment IS required. The partial pressure of a gas is defined as the total pressure of the system times the mole % of the ga,seous component. Where ( 02 is present with free water, a partial pres.sure ot. hi psia or greater would indicate that CO2 corrosion should be expected. If CO2 is not removed, inhibition and special metallurgy may be required. Below 15 psia, COt corrosion is not normally a problem, although inhibition may be required. 

The specifications for purified uranium and plutonium to be recycled are summarized in Table 21.7. Comparing these data with those presented before sliows diat at t 1 y the fission product activity must be reduced by a factor of 10 and the uranium contrait in plutonium by a factor of 2 X 10. The large number of chemical elem its involved (FPs, actinides and corrosion products) make the separation a difficult task. Additional complications arise from radiation decomposition and criticality risks and from the necessity to conduct all processes remotely in heavily shielded enclosures under extensive health protection measures. As a result reprocessing is one of the most complicated chemical processes ever endeavored on an industrial scale. 

Figure 16.4 Electrochemical corrosion cell used for testing the ennobling effect of a conducting polymer. The mild steel sample (1) is coated by a thin epoxy layer and is in contact with a 0.1 M UCIO4 solution (aerated or deaerated) through a small hole. Polymethylthiophene (PMT) is deposited on a Pt electrode (2) a saturated calomel electrode (3) and a Pt counter-electrode (4) are used with either (1) or (2) as the working electrode. Electrodes (1) and (2) are galvanically connected and can be disconnected from the electrical circuit for specific measurements. (Reprinted with permission from Electrochimica Acta, Corrosion protection by ultrathin films of conducting polymers by U. Rammelt, P.T. Nguyen and W. Plieth, 48, 9, 1257-1262. Copyright (2003) Elsevier Ltd)...

At above-room temperatures, the saturation index may become more positive, and, in any case, the rate of CaCOs deposition is higher, should there be any tendency toward deposition. Hence, corrosion protection at all temperatures without excessive scaling requires adjustment of water composition to a saturation index that is at least constant over the entire operating temperature range. Powell et al. [33,40] showed that, for any specific alkalinity, there is a corresponding pH value at which the decrease of measured pH with temperature is almost exactly compensated by a decrease in the factor (pA 2 Under these condi-... 

Many applications for bar, plate, and structural steel require use of painting or other protective measures therefore, there is not a great need for corrosion testing of the bare metal. An important exception to this, however, is the use of weathering steels, such as those specified in ASTM A 242, ASTM A 588, and ASTM A 852 in atmospheric exposure. The corrosion resistance of these steels is such that they can be used in the unpainted condition for many applications, depending on the environmental conditions. For these applications, there is a need for atmospheric corrosion testing to determine the suitability of bare weathering steels in specific locations. 

Design features also play an important role in the corrosion of embedded steel. Mix proportions, depth of cover over the steel, crack control measures and implementation of measures designed specifically for corrosion protection are some of the factors that control the onset and rate of corrosion. 

Cathodic protection is a useful supplement to other forms of water treatment, as a general corrosion inhibiting device in HW boilers, or where specific design configurations can lead to inadequately protected localized metal in steam boilers. Where BW makeup demands are minimal and boiler output is fairly constant, cathodic protection devices can also provide some measure of protection against hardness scales. Calcium carbonate salt is formed as a floc-culant or soft sludge rather than a hard scale, due to the peptizing effects of a zinc hydroxide complex formed from zinc ions in alkaline BW. 

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