Atmospheric Corrosion Chapter 3: Atmospheric Corrosion

Atmospheric corrosion Atmospheric corrosion is defined as the corrosion or natural degradation of material exposed to the air and its pollutants rather than immersed in a liquid. The rate or degree of degradation varies for different materials and is influenced by several environmental factors Many of these factors are natural in origin, but some result from man-made sources.

Types of Atmospheric Corrosion Atmospheric corrosion is classified in to 4 types Industrial atmospheres Marine Atmospheres Rural atmospheres  Corrosion under insulation (CUI)

Industrial atmospheres An industrial atmosphere is typically identified as one with heavy industrial manufacturing facilities. These atmospheres can contain concentrations of sulfur dioxide, chlorides, phosphates, nitrates, or other specific industrial emissions. These emissions combine with precipitation or dew to form the liquid corrosive.

Industrial atmospheres

Marine Atmospheres A marine atmosphere is laden with fine particles of sea salt carried by the winds and deposited on materials. The marine atmosphere is usually one of the more corrosive atmospheric environments. It has been shown that the amount of salt (chlorides) in the marine environment decreases with increasing distance from the ocean and is greatly influenced by wind direction and velocity.

Marine Atmospheres

Rural atmospheres  A rural atmosphere is normally one that does not contain chemical pollutants but does contain organic and inorganic dust. Its principal corrodent is moisture and, of course, oxygen and carbon dioxide. Arid or tropical atmospheres are special cases of the rural environment because of their extreme relative humidities and condensation. The rural atmosphere is generally the least corrosive.

Rural atmospheres

Corrosion under insulation Corrosion under insulation(CUI) is a severe form of localized external corrosion that occurs in carbon and low alloy steel equipment that has been insulated. This form of corrosion occurs when water is absorbed by or collected in the insulation. The equipment begins to corrode as it is exposed to water and oxygen. CUI is common in refineries and process plants that typically operate equipment at high temperatures.

Corrosion under insulation

Atmospheric Corrosion of Various Metals The average atmospheric corrosion rates of various metals for 10 year and 20 year exposure times are presented in table below. Corrosion rates are given for several metals at urban industrial, marine, and rural locations. Steel is the most common metal exposed to atmospheric corrosion. Other important metals include aluminum, copper, lead, tin, nickel, and zinc.

Atmospheric Corrosion of Various Metals Aluminum has high corrosion resistance in rural environments and lower resistance in industrial and marine environments. Nickel and nickel alloys have high corrosion resistance in marine and rural environments but can exhibit attack in industrial environments containing higher levels of sulfur compounds. Zinc also exhibits its highest corrosion rates in industrial environments, with somewhat lower rates in marine environments and still lower rates in rural environments.

Atmospheric Corrosion of Various Metals

Effects of Moisture and Atmospheric Pollutants Because atmospheric corrosion is an electrolytic process, the presence of an electrolyte is required. This should not be taken to mean that the steel surface must be awash with water; a very thin, adsorbed film of water is all that is required. During an actual exposure, the metal spends some portion of the time awash with water because of rain or splashing and a portion of the time covered with a thin adsorbed water film.

Effects of Moisture and Atmospheric Pollutants The portion of time spent covered with the thin water film depends quite strongly on relative humidity at the exposure site. This fact has led many corrosion scientists to investigate the influence of the time of wetness on the corrosion rate. These studies have shown that the time of wetness, although an important factor, cannot be considered in isolation when estimating corrosion rates.

Effects of Moisture and Atmospheric Pollutants

Effects of Moisture and Atmospheric Pollutants An example of this fact is demonstrated in the figures below, in which the weight loss of iron is plotted as a function of relative humidity for an exposure of 55 days in an atmosphere containing 0.01% sulfur dioxide (SO2). In the lower righthand corner, is the measured corrosion rate of iron exposed for the same time in an SO2-free atmosphere at 99% relative humidity. The increase in corrosion rate produced by the addition of SO2 to the atmosphere is substantial.

Effects of Moisture and Atmospheric Pollutants

Effects of Moisture and Atmospheric Pollutants When metals exposed in to dry atmosphere, they corrode at a negligible rate because of the absence of moisture to serve as an electrolyte. For example, 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.

Effects of Moisture and Atmospheric Pollutants Old truck exposed to dry atmosphere in desert

Corrosion Control Atmospheric corrosion is controlled by selecting more corrosion resistant materials. For example, aluminum or stainless steels can be used where the corrosion of steel parts is undesirable. Metallic and organic coatings are commonly used on steel to provide atmospheric corrosion resistance. The metallic coatings can be sacrificial, such as zinc coating on galvanized steel, or they can be a noble metal coating that acts as a barrier. Removal of moisture from the atmosphere to levels below the critical relative humidity will control corrosion. Air conditioning is commonly used to control the relative humidity in the atmosphere for protection of communications equipment and computer components.