Environment road salts

Such rusting phenomena as these are distressingly familiar in marine environments or in moderately cold climates where salts are used to deice roads. Acceleration of corrosion by seawater or sea spray, or by road salt, has several origins ... 

Sodium is present in soils and water as NaCl. In arid environments, NaCl accumulates in the surface and groundwater owing to irrigation and high evapotranspiration. Other activities such as road salting and water softening may also contribute NaCl to natural waters. For additional information on human health effects and drinking water levels, see Shelton (1989). 

Two major sources of salinity are important in the urban environment sewage and road salt. The salinity of domestic wastewater is derived from both the salinity of the source water supply to the municipality and the salts added directly by humans (Figure 11). This includes the use of detergents, washing powders, and salts. In Israel, for example, the average net human contributions of chloride, sodium, and boron to domestic wastewater are 125 mg, 120 mg, and 0.6 mg. 

Other electrical equipment is either directly buried or installed in underground conduit systems. In this type of environment, the extremes of temperature are less often experienced however localized and transient heating is experienced when power loads are high or when equipment runs near steam pipes for example. In underground installations, corrosive conditions are found based on road salts, lime extraction from concrete, and other minerals that become concentrated as water dries away. Exposure to microbes, oil run-off, decaying vegetation and other materials suggests that very stable polymers are required for this service. 

Figure 5 compares the ions that might be attributed to road salt use and acidic deposition in the northern cities. These include sodium, calcium, chloride, and sulfate they were major constituents. While the northern cities have expectedly high levels of these ionic species, the levels in Dallas were unexpected. Further observation pointed out important differences in the chemistry of the environments as described below. 

The high levels of calcium sulfate in Montreal are indicative of the heavy use of road salts, especially calcium chloride, and of the acid deposition that affects that area. Field corrosion tests showed how aggressive this environment can be even when the solution can quickly run off and evaporate lab tests indicate that after dying out and reacting with calcium chloride, the acid deposition may no longer be important to the corrosion process unless it is renewed. 

Simple laboratory tests may not be adequate to reproduce field results. The reason for this was demonstrated in the lab with an artificial solution containing road salt and acid rain constituents. Allowing such a solution to evaporate drives off the volatile nitric acid component and hydrochloric acid formed when calcium sulfate precipitates. Hence the history of a poultice is important in determining the aggressiveness of a particular environment. 

Results in Table I show that MIST test road salt spray alone did not produce blush and bloom although pitting was observed at the higher salt concentrations. Acid precipitation had a very pronounced affect at or below pH=4. This pH range is easily obtained on automobiles due to a drop in pH of the solution as a result of evaporation as well as the fact that the pH of precipitation has been less than 4 on many occasions. The appearance of the test panels was similar to that observed on vehicles after one or two years in Montreal or after several years in milder environments such as Detroit or Boston. 

The combination of acid precipitation with road salt spray produced the worst effects on anodized aluminum. As the MIST test pH became more acidic and the amount of salt increased the time to achieve an equivalent milky white appearance was reduced significantly (Table I). At more neutral high chloride concentrations severe pitting occurred while at more acid high chloride concentrations blush and bloom predominated. The latter environment is similar to that existing in the Northeast USA and Canada and, therefore, the results can explain the problems of blush and bloom in these areas. Corrosion surveys by automotive companies and trim producers in these areas have shown that blush and bloom and pitting have become increasingly more severe over the last ten years.5 These problems have led to a shift away from anodized aluminum as an automotive trim material in recent years. 

Auto body corrosion adjacent to stainless steel trim occurs in environments where de-icing salts are used. The effects of acid deposition combined with road salts produces a synergystic effect on the degradation of automobiles. (Photo by Robert... 

SECTION 20.8 Electrochemical principles help us understand corrosion, undesirable redox reactions in which a metal is attacked by some substance in its environment The corrosion of iron into rust is caused by the presence of water and oxygen, and it is accelerated by the presence of electrolytes, such as road salt. The protection of a metal by putting it in contact with another metal that more readily undergoes oxidation is called cathodic protection. Galvanized iron, for example, is coated with a thin layer of zinc because zinc is oxidized more readily than iron, the zinc serves as a sacrificial anode in the redox reaction. 

Electronics are exposed to a wide range of outdoor and indoor environments generally considered as atmospheric exposure. The corrosion behavior is determined by the actual environment, which can be as benign as a simple low humidity, purified atmosphere, indoor location, to the aggressive environment existing at a pulp and paper mill or on an automobile, which is subjected to road salt splash and spray. The electronics design as well as the nature of the environment are important because failures in printed circuit boards, integrated circuits, and other components have been known to occur even in extremely low levels of moisture and contaminants. Electronics components are mostly indoor or sheltered from direct exposure to liquid splash, spray, rain, snow, etc., and therefore the environment is considered atmospheric exposure. 

Automobiles and mUitaiy equipment frequently are exposed to marine environments and road salt. Beach test sites are available, but simulations in the laboratory are convenient. The ASTM B 117, Method of Salt Spray (Fog) Testing, has been widely used for this purpose to evaluate rusting, pitting, and SCC. Martensitic stainless steels and maraging steels have been exposed to salt spray (and other NaCl environments) to evaluate resistance to SCC prior to use in military equipment and fasteners. Ferritic and austenitic stainless steels have been tested to evaluate resistance to rusting prior to use in automotive applications. 

Gertler A, Kuhns H, Abu-Allaban M, Damm CR, Gillies J, Etyemezian V, Clayton R, Proffitt D (2006) A case study of the impact of winter road sand/salt and street sweeping on road dust re-entrainment. Atmos Environ 40 5976-5985... 

Corrosion of reinforcing steel in concrete (elevated highway. Providence, RI), where road de-icing salts combine with acid precipitation to produce a severe environment. (Photo by Robert Baboian.)... 

The section on metallic corrosion clearly indicates that the conventional method of classification of environments into marine, industrial, and rural no longer is adequate. More specific information is needed about the actual chemical components in the atmosphere as well as humidity and other factors. Specific environments also are addressed in the metallic corrosion section. For example, the automotive environment in the northeastern United States is particularly severe because of the combination of acid deposition and the use of road de-icing salts. These factors exert a synergistic effect on the corrosion behavior of auto-body steel and on exterior anodized aluminum automobile trim. 

The other main source of chloride in concrete is penetration from the environment. This occurs, for instance, in marine environments or in road structures in regions where chloride-bearing de-icing salts are used in wintertime. 

Silicone gels have also been used effectively to protect automotive electronic modules in the severe environment under a vehicle bonnet, where hazards include high temperature, water and salt from road surfaces. 

Chloride ions initiate the corrosion of steel by destroying the passive layer. The greater the chloride concentration, the more rapid the depassivation of steel. Chloride ions enter concrete as a contamination of raw materials used for the production of concrete (mainly water and sand) and as the result of diffusion from the surrounding environment especially in the case of applying salt to roads. The high aggressiveness of chloride ions is explained by the autocata-lytic course of the ionization reaction of steel in their presence... 

Atmospheric contaminants often responsible for the rusting of structural stainless steels ate the chlorides and metallic iron dust. Chlorides can originate from concrete (CaCl2) and spraying of salt on the road or from exposure to industry and marine locations. Chlorides promote pitting or crevice attack on stainless steels. As discussed above, the corrosivity of different atmospheres can differ and must be considered when materials are selected. Rural and urban environments without pollutants (chloride) do not corrode stainless steel, even in areas with high humidity. 

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