Steel 12Kh18N10T Crevice corrosion under heat transfer conditions

Abstract

During heat transfer from metal to solution, local metal activation occurs more readily within a crevice than under isothermal conditions, i.e., both for metal open surface temperature and also for crevice bottom temperature. Metal within a crevice is activated more readily the higher the heat flow density. Disruption of passivity is more likely to occur in the form of a corrosion spot close to a crevice bottom.Local corrosion is a typical form of heat exchanger equipment breakdown [1], and this is connected both with structural features of the majority of heat exchangers, and with their operating conditions. For contemporary heat exchanger equipment, heat exchange surfaces are typically made in the form of pipes, plates, and spiral baffles. With any method for finishing the ends of pipes in pipe grids, joints of plates with each other and spiral baffles with end caps, within heat exchangers there is formation of a considerable number of crevices and gaps, and this may lead to rapid crevice corrosion [2][3][4][5][6]. In the case of shell-and-tube heat exchangers, 61.4% of forced stoppages are caused by pipe breakdown and 30.6% are caused by tube plate failure [7]. Flange joints are especially vulnerable to crevice corrosion.Crevice corrosion is observed most frequently in heat exchange equipment made from passivating materials (stainless steels, titanium, etc.) in media containing ion-activators [2,5,8]. This problem acquires particular importance taking account of the increasing requirement of industry and power generation for recycled and desalinated sea water.The rate of crevice corrosion depends on solution pH value, oxygen and chlorides dissolved within it, and also on solution movement rate and its temperature [8]. Crevice geometric parameters [9, 10], and the ratio of surface area within and outside a crevice [11,12], have a considerable effect on crevice corrosion. Narrower crevices reduce metal endurance [10]. With an increase in crevice width, there is an increase of corrosion within a crevice, but with a crevice width of more than 3 mm this parameter ceases to affect corrosion [13]. With a crevice width less than the diffusion layer thickness, the limiting flow of oxygen regeneration, which determines the metal dissolution rate, becomes constant and does not depend either on pH value or on chloride concentration in solution [14].Heat transfer conditions, affecting metal surface temperature and the layer of solution at an electrode, and the rate of transport processes close to a metal surface, should have a marked effect on crevice corrosion. However, this aspect of crevice corrosion has hardly been studied. In [8], a number of factors influencing tube and tube grid crevice corrosion have been studied as applied to heat exchanger operating conditions, cooling sea water. Research was carried out on a model heat exchanger, and this made it impossible to establish quantitative features of crevice corrosion occurrence and development with heat exchange under strictly controlled conditions. In our view, the ...

Effect of heat flux on metal corrosion in non-boiling media: testing system, heat transfer simulations, and corrosion study on Al alloy

“…Most of the research on the corrosion of heat exchangers concerned the corrosion behaviour of metals [16][17][18][19] and the corrosion prevention effect of inhibitors [20,21] or other protection methods [22][23][24] under specific conditions encountered in the practical fields. In these studies, the effects of heat flux and T M were usually combined, ignoring the effect of heat flux.…”

“…Figure 1. Schematics of the testing system: (1,15) thermostat water bath; (2,22) glass container; (3) silicone tube; (4) water chamber; (5) test chamber; (6) metal sample; (7) salt bridge and reference electrode; (8,19) rubber O-ring; (9) counter electrode; (10) metal fixture; (11,21) circulating pump; (12,20) flow metre; (13) electrochemical workstation; (14) sealing cover;(16,17,18,23,24) thermocouple.…”