Atmospheric corrosion

The metals ordinarily used in equipment and structures corrode at a negligibie rate when exposed in to the atmosphere in the absence of moisture to serve as an electrolyte. For exampie, metal parts exposed in the desert air remain free from corrosion for long periods of time. Also, metal parts exposed to the air at temperatures below the freezing point of water or of aqueous condensates on the metal do not corrode to a significant extent, because ice is a poor electrolytic conductor.

Rural atmospheres are generally free of le particulate matter and corrosive gases found i industrial atmospheres, and this condition ccounts for the low corrosion rates in rural tmospheres. The normal carbon ioxide content of air has little or no effect on metals exposed to the atmosphere.

Industrial atmospheres contain dust as a rimary contaminant, in concentrations of bout 2 mg/m3 for average city air to 1000 mg/m3 for heavily industrialized areas. It is stimated that more than 35.000 kg of dust/km2 settles in urban idustrial areas in a month. Dust deposited on metal surfaces in these areas generally contains particles of carbon and arbon compounds, metal oxides and metal salts (chiefly sulfates and chlorides), and sulfuric acid. The combination of moisture with dust articles bearing soluble contaminants produces crevice corrosion by forming differential aeration cells and other types of concentration cells. Most of the soluble contaminants are hygroscopic and absorb moisture from the air when the relative humidity is substantially less than 100%. The critical level of relative humidity (that at which moisture absorption takes place on metals exposed to even relatively mild industrial atmospheres) is usually about 50 to 70% for steel, copper, nickel, zinc, and most metals that are used in structures and objects intended for industrial atmospheric exposure. This takes into account the effect of the normal fluctuations in temperature between day and night.
Such gases as sulfur trioxide, sulfur dioxide (which readily oxidizes to sulfur trioxide), hydrogen chloride, oxides of nitrogen, hydrogen sulfide, and the halogens in the air accelerate the crevice-corrosion effects of moist dust deposits on these metals. Even at low concentrations, such gases also corrode these metals in the absence of dust deposits, if the humidity reaches or exceeds the critical level.

Metal surfaces located where they become wet but where rain cannot wash the surfaces may corrode more rapidly than if fully exposed. For example, the rusting of steel in partly sheltered locations in moist air containing oxides of sulfur, which form sulfuric acid, is apparently accelerated in a self-perpetuating sequence of reactions. The acid attacks the steel, producing iron sulfate that is retained on the moist rust and hydrolyzes to form more sulfuric acid and iron oxide, thus catalyzing the rusting process.

Marine Atmospheres. At and near the seacoast, the deposition of salt-water spray is the most corrosive aspect of marine atmospheric exposure. The rate of attack on exposed metals varies widely, depending on distance from the sea, prevailing wind direction, relative humidity, and temperature fluctuations that can produce condensation.
The penetration of protective film by chloride ions and the high solubility and hygroscopic nature of metal chlorides cause rapid corrosion on carbon and low-alloy steels. Zinc and cadmium plating extend the life of steel hardware to a useful but limited extent; large steel structures must be protected by painting.
The high conductivity of moisture that contains dissolved salt accelerates crevice corrosion and galvanic corrosion, making the use of sealants mandatory at joints and the use of single-metal systems good practice.
Brasses undergo rapid dezincification unless alloyed with small amounts of arsenic, antimony or phosphorus in inhibited grades. Copper nickels, titanium, perform well in marine atmospheres, but most metals corrode severely unless protected by organic coatings.
Brasses undergo rapid dezincification unless alloyed with small amounts of arsenic, antimony or phosphorus in inhibited grades. Copper nickels, titanium, perform well in marine atmospheres, but most metals corrode severely unless protected by organic coatings.
In general structure applications of aluminum alloys and stainless steels in thick sections, pitting and crevice corrosion are not usually serious problems. Pitting and crevice corrosion can produce rapid penetration and failure by leakage in thin-wall vessels, tubes, and pipes made of these alloys.
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