2.16 - Atmospheric Corrosion

Abstract

Like all other corrosion, atmospheric corrosion is an electrochemical process, which is controlled by oxidation and reduction reactions. With atmospheric corrosion, these electrochemical processes occur in a local environment that undergoes changes because of diurnal and seasonal cycles and because of the growth of surface oxide layers. The progression of events on a freshly exposed metal surface follows a typical course, with an immediate hydration of the surface, followed by periodic formation of moisture layers on the surface (moisture drops or layers may form because of the condensation or deposition of rain, fog, or wet aerosols, which in turn may be removed by evaporation or cleaning), and then reactions within these moisture layers that may lead to the breakdown of the hydrated oxide layer (e.g., ligand exchange, oxide dissolution) or to the build up of oxide layers (oxide growth and precipitation reactions). This chapter will explore how the environment controls the formation and persistence of moisture layers, and how it controls the chemistry within such moisture layers (wet deposition, particulate deposition, gaseous absorption). It will commence with an overview of how the atmosphere regulates the composition of rain droplets, particulates, and atmospheric gases, and the typical range of compositions of each phase. The chapter will then address the transport and deposition of each phase, and the moisture layers that form either directly or indirectly by this deposition. The time history of the different forms of moisture will be discussed in relation to the local climate (surface temperature, relative humidity (RH), etc.) The nature of chemical and electrochemical processes occurring in the moisture layer and the formation of oxide layers will be the last area addressed.