Corrosive Behaviour of Super alloys in Atmospheric conditions
Atmospheric corrosion cannot be clearly defined. It includes immersed corrosion and dry oxidation as metals can be exposed to damp exposure or may be exposed to the full force of weather. It usually includes packaging, storage, painting and effects of climate and air purity.

Materials exposed to uncontrolled normal exposures may receive corrosion faster and by various mechanisms than those placed in pure, dry air — even if they are not subjected to rain. In dry conditions, the growing oxide layer often secures the underlying metal, giving rise to a logarithmic or square-root time law. In uncontrolled conditions, the rate often remains stable for a period, however may drop off when the layer has grown to a suitable thickness. If the metal is exposed to rain, it may corrode while it is wet at the rate suitable to immersion in impure, well-aerated water, however the rate decreases in dry atmosphere.
Corrosion in Moist Conditions
Corrosion rates of metals are higher in humid conditions even in pure air and it further increases in presence of sulfur dioxide.

An effect of moist atmosphere is to setup drops of strong electrolyte with classical pattern of corrosion with a small anode in the centre of drop acting as a source of ferrous ion and oxygen reduction cathodes at the edges developing hydroxyl ion. These drops become covered by a transparent skin, presumably of ferrous hydroxide that darkens and thickens and eventually runs through the hydrated ferric oxides. In atmospheric conditions, these drops may remain as discrete scabs or may spray or develop tracks that move throughout the surface. When the rust layer is completed, the metal surface may lack of oxygen, and it is very common opportunity to develop a layer of Fe3O4 under the hydrated rust. The availability of corrosive salts will prevent this layer from getting very secured.
Iron and steel show rather similar phenomena on exposure to uncontrolled conditions. Copper develops a green film of corrosion product containing hydrated sulfate or carbonate or chloride. Aluminum shows nominal rates of corrosion in various conditions however may get attacked quickly when subjected to alkaline solutions.
Types of Atmospheric Corrosion
Different factors influence the corrosion properties of an atmosphere. Various factors such as relative humidity, temperature, sulfur dioxide concentration, hydrogen sulfide concentration, chloride content, extent of rainfall and dust and also the angle of exposed metal, all have an influence in corrosion behaviour. In dry environments, free of contaminants, only nominal corrosion would occur.
A general practice is to categorize atmospheres such as rural, industrial and marine. The industrial atmosphere contains variety of contaminants, however contamination by sulfur is crucial. Burning of fossil fuels produces large magnitudes of sulfur dioxide that is transformed to sulphurous acid and oxidized to sulfur trioxide in the available of air and moisture. The corrosivity of an industrial atmosphere diminishes with distance from a city due to reduced degree of sulfur dioxide.
Marine conditions extremely corrode several metals. The atmospheric test lab shows that steels subjected at 80 feet from ocean are corroded 10 to 15 times faster than steel subjected to ocean at 800 feet.
When exposed indoors, Nickel 200 barsremains bright and free of tarnish, offering superior performance than silver, copper and brass. Outside, it loses its color, attaining a thin adherent corrosion layer. The rate of corrosion is very slow, however corrosion rate increases with increase in sulfur dioxide concentration of the atmosphere. Marine conditions and rural conditions are equivalent for their effects on Nickel 200 causing nominal corrosion rates.
Corrosion resistance of Inconel bar is excellent in indoor atmospheres even in damp sulphurous conditions that may affect Nickel 200 and Monel 400. In rural atmospheres, outdoors, the alloys retain brightness for several years however may create a nominal tarnish in industrial, sulfur bearing conditions. Free exposure to the atmosphere is more favourable than sheltered exposure in which the beneficial effects of rain in washing the surface and sun and wind in drying doesn’t occur. Incoloy 800 develops a very thin oxide layer and nominal pitting after extensive exposure to marine conditions.

The Iron-Nickel-Chromium-molybdenum alloys and Nickel-Chromium-Molybdenum alloys remain bright and tarnish-free in rural and industrial conditions. They also retain their brightness in marine conditions. Even corrosion resistance by Hastelloy C-276 bars is excellent in all types of atmosphere conditions.
Corrosion rate of Monel 400 bar is nominal in all types of conditions. Indoor exposure develops a light tarnish that is easily eradicated by occasional wiping. Outdoor surfaces that are exposed to rain create a thin gray-green patina. In sulphurous conditions, a smooth, brown, adherent layer develops. Due to its low corrosion rate and the pleasing patina that creates, Monel 400 is used for architectural service as roofs, gutters and flashings.
