Corrosion design factors

For economy in construction and operation of plants for handling saline water, particular attention must be paid to corrosion factors. Steel can be used in many applications, if it is protected by the use of coatings or is given cathodic protection. Allowance also can be made in the design for the corrosion of steel. If it does not conflict with other design factors, incoming sea water should be deaerated to control steel corrosion. Corrosion inhibitors and chemical treatment may be employed to reduce the attack, but frequently this method is too costly. 

Corrosion prevention involves inherent factors, which are within the control of the metallurgist or engineer. The three main categories considered are materials selection, design factors and life prediction analysis. 

Corrosion protection refers to a situation in which all the inherent factors to prevent corrosion have been optimized and external intervention is deemed necessary to minimize the corrosion that is beyond the scope of corrosion-preventative factors such as metallurgical, design and life prediction analysis. Corrosion protection can be achieved by (i) addition of inhibitors and (ii) use of protective coatings. 

In the past, specifically prior to the required installation of SO2 scrubbers, most chimneys were subjected to hot, dry, seldom acidic conditions. Typical flue gas temperatures exceeded 400°F and were therefore above the acid dew point during normal plant operation. Furthermore, the plant itself experienced fewer shutdowns due to the intermittent operational difficulties inherent within the scrubbed gas systems themselves. Thus, chimney linings were not exposed to severe acidic conditions other than at infrequent start-ups and shutdowns, For this reason, wet acid corrosion was not a major design factor, and chimneys were relatively simple to design, construct and maintain. There were a few relatively common designs, which will be briefly described. 

Takeuchi, N. and Fuller, T.F. (2008) Modeling and investigation of design factors and their impact on carbon corrosion of PEMFC electrodes. J. Electrochem. Soc., 155 (7), B770—B775. 

Lifetime modeling and prediction, design for specific corrosion properties, quantitative environmental corrosion Intensity factor / / / / / /... 

The cumulative effect of an anodic ORR (induced by O2 crossover from the cathode to the anode) on cathode carbon catalyst-support loss (C corrosion) under fuel starved and ordinary (e.g. constant current) PEMFC operating eonditions were studied.The effects of C corrosion at constant cmrent are less severe than start-up and fiill H2 partial starvation, but they are large enough to affeet the cell performance after a long-time operation. The design factors of the MEA and operational factors such as humidity and temperature also affect carbon loss. The influence of these parameters is not always simple, and the coupling of these factors was addressed with MEMEPhy s to elucidate the rate of carbon loss under normal PEMFC operation... 

Other chains must operate in very corrosive conditions. Many such cases may be found in fertilizer, chemical, paper, and food processing plants. Chains that are made to work in these conditions normally are made from special materials, and some design factors may be modified. 

Design factors which the NiMH manufacturer must consider for good shelf life are the oxidation and corrosion resistance of the metal hydride alloy, the amount of precharge on the metal hydride electrode, the nickel hydroxide active material formula, and the quality of the cobalt conductive network in the positive electrode. 

Fig. 10.22 The influence of design factors on corrosion, (a) Storage tanks should be fabricated to allow complete drainage and access to outside surface for painting, (b) The insulation of cool pipe supports may prevent internal or external condensation of an electrolyte film. Insulation may prevent bimetallic corrosion tight insulation may prevent water ingress, (c) Full length, continuous welds are preferred to intermittent, tack ones. Slag and surface oxides must be removed prior to surface coating.

Table 10.5 A summary of design factors which are important in corrosion protection...

Corrosion inhibitor research requires accelerated testing of candidate inhibitors such as REM compounds, before field trial of the most successful candidates. A challenging issue is that an inappropriately designed accelerated inhibitor test could introduce major uncertainties to its results. Acceleration of a corrosion test is usually achieved through the enhancement of the aggressiveness of the test environment in order to intensify major corrosion controlling factors. The identification of major environmental factors that may control the thermodynamics, kinetics and mechanism of a corrosion process is, therefore, the first step in inhibitor test design. 

Several tests are not related to any particular part of the corrosion process, but involve only a specific test specimen that responds to corrosion by complete failure. These tests are used in the measurement of certain forms of corrosion involving factors such as stress. Examples are corrosion fatigue, stress corrosion cracking, and hydrogen embrittlement In designing such corrosion tests, the variety of test specimens parallels the number of apphcations. 

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