Corrosion of Iron Under Alternating Wet and Dry Conditions

• Published in: Corrosion (Houston, Tex.) Vol. 56; no. 5; pp. 470 - 481
• Main Authors: Dunn, D.S., Bogart, M.B., Brossia, C.S., Cragnolino, G.A.
• Format: Journal Article
• Language: English
• Published: Houston, TX NACE International 01.05.2000; NACE
• Subjects: Alternating current; Analytical methods; Applied sciences; CHLORIDES; CORROSION; Corrosion potential; Corrosion products; Corrosion rate; Corrosion tests; Direct current; EARTH ATMOSPHERE; ELECTROCHEMISTRY; Electrolytes; Evaporation; Exact sciences and technology; HUMIDITY; Ionic strength; IRON; IRON OXIDES; Magnetite; MATERIALS SCIENCE; Metals. Metallurgy; Purity; RAMAN SPECTROSCOPY; Thickness; Weight loss measurement; Wetting; Iron oxide; Corrosion rate; Transition metal; Dry; Iron; Experimental study; Corrosion product; Corrosion resistance; Corrosion; Corrosion test; Wet air; Raman spectrometry; Atmospheric corrosion
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3280551

ABSTRACTAtmospheric corrosion of engineering structures such as buildings and bridges is strongly dependent upon exposure conditions. Atmospheric corrosion tests have been conducted over many years to make long-term predictions.1 These tests have shown that corrosion rates of engineering steels are lower in lowhumidity rural environments and increase in high-humidity marine environments. Corrosion of steel pilings is many times faster in tidal splash zones compared to marine atmospheres (i.e., out of water and several hundred feet inland from shore) or continuously immersed conditions.2 The atmospheric corrosion rate of carbon steel is determined by the relative humidity (RH), pH, and pollutants in the environment. Similar to most metals, iron and steel have been reported to have a primary critical RH of ~ 60%, above which a liquid film formed by several monolayers of water develops on the surface.3 Above this threshold, corrosion governed by electrochemical laws proceeds at a slow rate. At 75% to 80% RH, the corrosion rate sharply increases. This secondary critical RH has been attributed to capillary condensation of water in the pores of the iron corrosion products.3 The deposition of hygroscopic salts on the metal surface can decrease the critical RH and increase the corrosiveness of the electrolyte. In aerated environments, the corrosion rate of carbon steel is controlled by the reduction of oxygen:O2 + 2H2O + 4e­ 4OH­