Unalloyed atmospheric corrosion

It is agreed generally that the characteristics of the rust films that form on steels determine their resistance to atmospheric corrosion. The rust films that form on low-aUoy steels are more protec tive than those that form on unalloyed steel. 

Urban or industrial atmospheres are those atmospheres that are high in CO and CO2, sulfates and sulfites, and possibly various N,0 gases. The corrosion rate is largely the same for unalloyed aluminum as with most conventional aluminum alloys (alloys with Zn, Mg, Si, and/or Mn). Aluminum alloys with copper will exhibit corrosion rates 4-20 times higher. When the SO2 concentration is less than 0.01% by volume, the atmospheric corrosion rate, even at a relative humidity of 98%, is negligibly affected by the presence of the SO2. This is probably due to the low adsorption tendency for SO2 on aluminum surfaces. When the SO2 concentration exceeds 0.01% by volume, severe corrosion effects occur. 

Above a relative humidity of -70% the corrosion of unalloyed and low-alloy steels increases rapidly. In the absence of additional pollutants the atmospheric corrosion of iron is described by the electrochemical reaction ... 

It is known that addition of small amounts of copper to unalloyed steel can reduce atmospheric corrosion (Fig. 1-29 Drodten, 1969). A special group of alloys which should be mentioned are the weathering steels or COR-TEN steels developed at the beginning of the 1960s. By alloying the steel with elements like Cu, P, Cr, Si and Ni corrosion resistance in outdoor atmospheres is improved by the presence of protective layers. The rust which develops on weathering steels is more protective than that on unalloyed steel. In principle, the corrosion... 

The influence of the alloying elements - of chromium as well - up to a total level of about 5% is described in detail in Section Unalloyed and low-alloyed steels/cast steel . Steels with alloy contents at this level must be protected against corrosion just like unalloyed steels in the splash zone and tidal zone. Despite the much lower corrosion rate in the atmosphere in these materials, the use of weather-resistant steels without corrosion protection cannot be recommended in sea air either. 

The corrosion resistance of stainless steels with much less than 17% chromium is insufficient to maintain an acceptable surface quality when exposed openly to the weather in marine atmosphere or in seawater over the longer term. These materials are, however, much more resistant to corrosion than the unalloyed and low-alloyed steels. They are used in atmospheric exposure when there is an emphasis on... 

The corrosion mechanism for weathering steels is similar to that of unalloyed carbon steels. The rust forms a more dense and compact layer on the weathering steels than on unalloyed carbon steels. The rust layer more effectively screens the steel surface from the corrosive envirorunents of the atmosphere. The corrosion process may be affected in several ways by this rust layer. The cathodic reaction may be affected by the low diffusion rate of oxygen, whereas the anodic reduction may be retarded by limiting the supply of water and corrosion-stimulating ions that can reach the surface of the steel. In addition, the increased electrolyte resistance may also decrease the corrosion rate. 

While the corrosion rate of bare steel tends to decrease with time, the difference in corrosivity of different atmospheres toward unalloyed cast irons or steels can be quite dramatic. The relative corrosivity for open-hearth steel in atmospheres ranging from a desert to the spray zone on an ocean beach is shown in Table 9.5. Similar ranges in corrosivity were determined by the ISO 9223 corrosion rates for steel (Table 9.1). In a few cases, the corrosion rates of ferrous metals have been reported as increasing with time, and careful analysis of the exposure conditions generally reveals that an accumulation of contaminating corrosive agents has occurred, thus changing the severity of the exposure. 

Sulphur has a negative effect on the conductivity of unalloyed copper and - just like oxygen - causes at higher temperatures in an hydrogen atmosphere the formation of pores resulting in a decrease in strength. The latter is also valid for some copper alloys. Moreover, sulphur has a negative effect on the corrosion resistance of copper. 

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