Rust Appearance

Uncoated panels were exposed at the sites and were drawn after 18 months (m) and 42 months (m) to check the surface appearance. The details of the same are given in chapter 4(results). 

The exposed uncoated MS and WS panels were drawn from the sites (PI, P2 and P3) from time to time and corrosion rate was calculated as per mass loss method using standard [1]. 

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In ancient times, many societies interpreted crop failure as a sign of retaliation from some higher power. Wheat rust, for example, was such a problem for the Romans that they created a god of rust. Each spring, before the rust appeared, the Romans held a festival to appease the god and ottempt to save their wheat crop. 

FIGURE 17.14 The corrosion of iron. Note that pitting occurs in the anodic region, and rust appears in the cathodic region. 

Figure 21-12 The corrosion of iron. Pitting appears at the anodic region, where iron metal is oxidized to Fe +. Rust appears at the cathodic region.

Tom Runyan s lOth-grade students at Monroe High School (Middletown, Ohio) put 5 X 5 cm squares of uncoated, cold-rolled steel in culture plates, each lined with a vinegar-saturated piece of paper towel. They cover the plates and wait to see what changes occur. Within two hours or so, rust appears on the steel, signalling corrosion. 

A high sulfuric acid content of industrial and urban atmospheres shortens the life of metal structures (see Tables 9.2 and 9.3). The effect is most pronounced for metals that are not particularly resistant to sulfuric acid, such as zinc, cadmium, nickel, and iron. It is less pronounced for metals that are more resistant to dilute sulfuric acid, such as lead, aluminum, and stainless steels. Copper, forming a protective basic copper sulfate film, is more resistant than nickel or 70% Ni-Cu alloy, on which the corresponding hlms are less protective. In the industrial atmosphere of Altoona, Pennsylvania, galvanized steel sheets [0.381 kg zinc per m, 0.028 mm thick (1.25 oz zinc per ft, 1.1 mil thick)] began to rust after 2.4 years, whereas in the rural atmosphere of State College, Pennsylvania, rust appeared only after 14.6 years [19]. 

The macrographs of rust formed on MS and WS after 42 months (m) of field exposure at PI are shown in Figs. 3.1 and 3.2, where both downward and skyward sides were considered. The rust macrographs of MS and WS after 42 m exposure at P2 and P3 are shown in Figs. 3.3 and 3.4, respectively. Rust appearance observed after 18 m and 42 m of atmospheric exposure has been listed in Table 3.1 for all the sites. 

In salt spray test rust appeared on the scribed lines of coated panels with progressive exposure of 1,000, 500 h. After 2,000 h, rust appeared on the scribed lines (<0.5 mm) on coated WS and increased (>1.5 mm) with 4,500 h exposure. 

Outdoor exposure tests by the Dutch Galvanizing Institute showed that on galvanized panels with 70-80 pm zinc scratched with widths up to 5 mm, no rusting took place after 8 years of weathering in an industrial-marine climate. Similar tests were made on cut galvanized rebars (diameter 6-20 mm), exposed for 5 years. On the 6-10 mm diameter bars, no rust appeared. On the 20 mm diameter bars, the cut surface showed a uniform, light brown layer of rust. However, the ru.st had not undercut the zinc coating. 

The SO4 ion is released again when H2SO4 dissolves in water. SO2 is the main contributor to atmospheric corrosion. Little rusting appears to take place in the absence of SO2. One of the best examples of how metallic objects are protected in the absence of SO2 is the good state of preservation of the famous Iron Pillar at Qutub Minar, in Delhi, India, which is 1600-years old. 

Although absence of any redox reactions is ideal for electrode lifetime, these reactions are very hard to avoid. As soon as the electrode materials come in contact with an ionic solution, lots of different reactions may occur, even if no current flows into the electrode. Consider a piece of iron immersed in saline solution. After enough time, rust appears on the iron surface. Corrosion occurs following simultaneous reduction and oxidation reactions on the surface. While the metal is oxidized, oxygen (O2) or H+ ions are reduced at the same time to complete the redox reaction. 

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