Materials Performance

NOV 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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47 NACE INTERNATIONAL: VOL. 53, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2014 the cable to allow annual certification of the cable. Deterioration Model To assist in the management of the sus- p ension cables, an analy tical tool was developed to model the deterioration of the galvanized steel wires from corrosion. While the present condition can be quanti- fied, an assessment of the mechanism and timescale is required for this condition to be established. Using this method, it is also possible to predict the long-term effects of various remedial measures. Details of th e model de velopm ent are descri b ed elsewhere. 2 To assist in the development of the model, it was necessary to measure a num- ber of physical parameters. In areas where corrosion had not initiated, the as-built condition such as wire diameter and thick- ness of galvanizing could be established. It was also possible to retrieve samples of wires for detailed inspection and physical testing to identify the factors that governed fai lure. In areas w h ere c orro sion had occurred, it was possible to measure sec- tion losses or depths of penetration that had resulted. Additionally, it was possible to identify failed wires and obtain values for contributory factors. For the wires making up the main sus- pension cables, the following assumptions were made with respect to the onset of cor- rosion: • The wires arrived on site adequately protected from corrosion until the cables had been spun. • Once in place, the cables were pro- tected by three layers: 4 The zinc galvanizing on the indi- vidual wires 4 A layer of red lead oxide paste on the outside of the cable 4 A protective wrap consisting of wire, tape, and coating • Initially, th e cable was protected from significant corrosion by the cu- mulative action of the three protec- tive systems. • The first protective layer to break down was the outer coating, which allowed moisture and, more impor- tantly, moist air to enter the bundle. As the cable cooled at night, the moisture in the air condensed to liquid. • In time, through exposure to water and the atmosphere, the effective- n ess of th e red lead past e broke down, allowing the zinc galvanizing to start corroding. • As patches of the zinc layer became fully consumed, the underlying steel started to corrode. • W hi l e th e rat e of c orro sion was i n i t i a l ly f a st , t h e g e n e ra t i o n of v o l u m i n o u s c o r r o s i o n p r o d u c t s eventually occluded the corrosion site, which slowed the rate of metal loss. • Under stress, the corrosion of the wires became concentrated, eventu- ally reducing the cross section of the wire sufficiently for it to fail by tensile overload. Tool Development Each of these stages needed to be mod- eled individually, based on both published data and site observations, then combined to produce the overall predictive tool. A series of laboratory investigations was car- ried out on samples of wire removed from the structure. The failure of the wires was found to be caused by the formation of nar- row "V "-shaped corrosion pits that reached a critical depth. The data on the critical defect size were best characterized by a Weibull distribution, which confirmed that failure occurred when the defect reached approximately one third of the thickness of the wire. Based on this observation, the most appropriate data for modeling sec- tion loss related to depths of penetration with respect to time. Published data were employed for the corrosion loss of zinc and unalloyed steel in a range of environments. 4 The data obtained from the site were found to correlate well with the section loss predicted by the model (Figure 2), and FIGURE 1 The Severn Suspension Bridge was built in 1966.

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