Weathering steel corrosion

Weathering Steel Corrosion. Triplicate specimens were exposed for periods varying from three to thirty months with exposures started at each of the four seasons during the first year. The experimental procedure and exposure schedule are documented in an EPA report (2f The results are 153 sets of triplicate weight loss data with site, exposure time and initial exposure season as primary variables. 

Table III. Regression Coefficients for Theoretical Model of Weathering Steel Corrosion C/t = i i ° i i/ p...

Figure 1. Fit of weathering steel corrosion data to model,...

Weathering steel corrosion can be described as competing mechanisms of formation and dissolution of a protective oxide layer during periods of wetness. 

Townsend, H.E. Atmospheric corrosion performance of quenched and tempered weathering steel. Corrosion 56, 883-886 (2000)... 

For weathering steels corrosion product accumulated with time leading to the retardation of corrosion rate with time, whereas for mild steel corrosion rate increased with time. 

Low-alloy steels, which contain about 2-3% of alloying elements, commonly copper, chromium and nickel. These steels still rust, but under certain conditions in the atmosphere, the rust formed becomes adherent and protective so that the corrosion rate becomes several times less rapid than with the ordinary steels mentioned above. These steels are often termed weathering steels. 

The improvement in rust resistance achieved through low-alloy additions obviously depends on the nature and amounts of the alloying elements — incidentally their effects are not additive — and to an even greater degree on the nature of the corrosive environment. To make a broad generalisation, weathering steels show to maximum advantage when they are freely exposed to the open air in industrial environments but, even then, their performance... 

An interesting development in weldable corrosion-resistant steels is the copper-bearing or weathering steels (Section 3.2) which exhibit enhanced corrosion resistance in industrial atmospheres in the unpainted condition. For optimum corrosion resistance after welding, the filler employed should be suitably alloyed to give a deposit of composition similar to that of the steel plate... 

Legault, Mori and Leckie have used open-circuit potential vs. time measurements and cathodic reduction of rust patinas for the rapid laboratory evaluation of the performance of low-alloy weathering steels. The steel specimens are first exposed for 48 h to the vapour of an 0-(X)l mol dm sodium bisulphite solution maintained at 54°C (humid SOj-containing atmosphere) to stimulate corrosion under atmospheric conditions. They are then subjected to two types of test (tt) open-circuit potential-time tests for periods up to 3 000 s in either distilled water or 0 -1 mol dm Na2S04 and... 

A relationship was also established between the oxide-reduction time and time of exposure, and the results for a mild steel and a lCu-3Ni weathering steel were similar to those obtained by mass loss. The authors give various expressions that relate oxide-reduction time (min) with corrosion rate (mm/y), and claim that a short exposure to a laboratory SO2 atmosphere followed by determining the E vs. time and oxide-reduction time provides a rapid method of evaluating weathering steels. 

J. R. (1991) The corrosion products of weathering steel and pure iron in simulated wet-dry cycles. Hyperfine Interactions 66 63—70... 

Galvez, N. Barr6n,V. Torrent,. J. (1999 a) Effect of phosphate on the crystallization of hematite, goethite, and lepidocrodte from ferrihydrite. Clays Clay Miner. 47 304-311 Gancedo, J.R. Grada, M. Marco, J.F. Pala-dos, J. (1988) Comparative Mossbauer and SEM study of the corrosion reaction of iron and weathering steel in SO2 polluted atmospheres. Hyperfine Interactions 41 637-640... 

The role of alloying elements in weathering steels consists of the effect of formation of the protective layer of corrosion products increase in mechanical strength and toughness and improved weldability. The protective qualities of the corrosion products on the steel depend on the continuous growth of the adherent, compact, inner layer and on low porosity within the layer. The kinetics of atmospheric corrosion were found to obey the equation,... 

Weathering steel data indicated rather constant corrosion rates after 8 year exposures (1967-1975) at marine, urban and rural sites in the eastern U.S. Although this data is limited, it suggests that the corrosion history of this weathering steel alloy must be relatively insensitive to both SO2 concentration and to the pH of rainfall. An alternative explanation is that pollutant levels above some threshold value are sufficient to induce the observed corrosion. Even so, the data do not offer clues as to the pollutant effect. 

Environmental Factors Affecting Corrosion of Weathering Steel... 

Weathering steel samples were exposed for periods of up to 30 months at nine air monitoring sites in the St. Louis, Missouri area. Climatic and air quality data were recorded during the exposure period and subjected to a rigorous evaluation to eliminate recording errors and to estimate missing values. Weight loss was used as the measure of steel corrosion. 

The coefficients in Table 3 provide a better theoretical understanding of how the different factors affect the corrosion of weathering steel. The 3 coefficients affect the solubility of the protective oxide layer and the a coefficients affect the diffu-sivity through the layer. The large ratio of a/3 confirms the relative insolubility of the rust on weathering steel in most environments. Sulfur dioxide increases the solubility of the film while NO2 and rain decrease the solubility. Rain apparently washes away acidic components (deposited during dew formation) that increase the solubility. 

Ozone appears to have no significant effect on the corrosion of weathering steel. 

During exposure, galvanized steel and weathering steel are expected to corrode by forming a protective surface film that retards corrosion. The rate of corrosion would then be diffusion controlled and depend upon the thickness of the film. Many of the corrosion products, particularly the carbonates, hydroxides, etc. within the film are soluble in acid solutions. In environments where increased acidity is present, dissolution of the protective film is a competing mechanism that accelerates the rate of corrosion of metals. 

Long-term corrosion studies of weathering steel indicate that the corrosion time function should be nearly parabolic ( )(Z) ... 

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