Atmospheric corrosion particles deposition

Wang Jia, Zhang Jibiao. The effect of micro-droplets formation caused by the deliquescence of the deposited salt particle on atmospheric corrosion of metals. Proceedings 16th International Corrosion Congress, Beijing, China, September 19-24, 2005. 

One excellent example of a multiana-lytical laboratory study is the influence of submicron sized particles of ammonium sulfate ((NH4)2S04) on the atmospheric corrosion of selected metals [22]. These particles were aerosolized and deposited under dry conditions on the metal surface, whereby the deposited amounts corresponded to up to 10 years of exposure in indoor locations of USA. By introducing humidity into the exposure... 

Atmospheric corrosion rates are commonly related to a critical relative humidity , above which the corrosion rate increases significantly and below which the rate is insignificant for many practical purposes. Depending on metal and exposure conditions, critical relative humidities have been reported in the range from 50 to 90%. The critical relative humidity is associated with the point of deliquescence of deposited aerosol particles, above which the aerosols rapidly absorb water until a saturated solution is obtained. For a single-phase aerosol, there is a well-defined critical relative humidity, whereas for a mixture of phases (the common situation in natural outdoor environments) the critical relative humidity is lower than those of the single phases. 

Atmospheric corrosion differs from the action that occurs in water or underground in that a plentiful supply of oxygen is always present. In this case, the formation of insoluble films, and the presence of moisture and deposits from the atmosphere become the controlling factors. The presence of contaminants such as sulfur compounds and salt particles also affects the corrosion rate. Nevertheless, atmospheric corrosion is mainly electrochemical, rather than a direct chemical attack by the elements. The anodic and... 

In atmospheres with chlorides present the corrosion of carbon steel proceeds in local cells that resemble the sulfate nests. They may form around chloride particles deposited on the surface, where the concentrated local chloride solution destroys the passivating film of FeOOH. The chlorides are concentrated in the anodic areas formed by migration, while the surrounding area covered by rust acts as a cathode. 

At an air temperature of 283 K (10 °C), an air pressure of 1,013 hPa and 60% relative humidity the water content is around 5.7 g/m. At 303 K (30 °C) and relative humidity of 100%, the water content rises to 31.4 g/m. The water film that condenses when the temperature drops or on relatively cold surfaces is always saturated with oxygen. Whereas corrosive action in a non-marine atmosphere is mainly determined by moisture content and potential industrial contaminations, the marine atmosphere is characterised by a raised content of salt particles carried on the wind from the sea spray. Since the salt particles deposited on the metal surface, or aerosols containing salt, also contain hygroscopic components, e.g. calcium and magnesium chlorides, liquid films form on the surface with very high salt content levels, even if the air is still above the dewpoint. 

There are other types of pores in a single solid particle. a-FeOOH is a precursor material for magnetic tapes, a main component of surface deposits and atmospheric corrosion products of iron-based alloys, and a mineral. The a-FeOOH microcrystal is of thin elongated plate [83],... 

In general for atmospheric corrosion, dusts and solid precipitates are hygroscopic and attract moisture from air. Salts can cause high conductivity, and carbon particles can lead to a large number of small galvanic elements since they act as efficient cathodes after deposition on the surface. The most significant pollutant is SO, which forms H SO with water. Water that is present as humidity bonds in molecular form to even the cleanest and well-characterized metal surfaces. ... 

Detailed studies have been imdertaken to explore the initial atmospheric corrosion of copper and zinc induced by particles, including NaCl and (NH4)2S04. They show a complex interplay between gases involved, such as SO2 and CO2, and the droplets around the deposited particles. The composition and lateral distribuhon of the corrosion products turn out to be governed by the location of anodic and cathodic sites, the transport of ions and other species between these sites, the deposition of gases onto the surface, and complicated spreading effects of the droplets formed [99]. 

The use of Ni-base superalloys as turbine blades in an actual end-use atmosphere produces deterioration of material properties. This deterioration can result from erosion or corrosion. Erosion results from hard particles impinging on the turbine blade and removing material from the blade surface. The particles may enter through the turbine inlet or can be loosened scale deposits from within the combustor. 

Similarly, SO2 and SO3 (SOJ compounds are produced in combustion by the oxidation of sulfur compounds within the fuel source. SO , emitted into the atmosphere can be incorporated into aerosol particles and wet-deposited as corrosive sulfuric acid. Both NO , and SO , emissions contribute to acid rain content from wet deposition, due to their participation in the formation of nitric and sulfuric acid, respectively. 

The atmosphere is presented on the top of the diagram with the surface water layer in the center. The corroding metal and the corrosion products are shown at the bottom of the figure. The wavy arrows indicate the oxidation of iron to Fe. The gas phases that enter the electrolyte are carbonyl sulfide (COS), hydrogen sulfide (H2S), and sulfur dioxide (SO2). Sulfate anions enter the electrolyte as part of the deposited atmospheric particles or as a component of precipitation. Several processes, such as the one oudined in... 

A marine atmosphere is laden with fine particles of sea mist carried by the wind to settle on exposed surfaces as salt crystals. The quantity of salt deposited may vary greatly with wind velocity and it may, in extreme weather conditions, even form a very corrosive salt crust, similar to what is experienced on a regular basis by sea patrolling aircraft or helicopters [Figs. 9.3(a) and (fc)]. 

Contamination of copper suifaces with atmospheric particulate matter can accelerate the corrosion process. For example, the corrosion of copper in 100°C air in the presence of submicrometer ammonium sulfate particles has been found to depend strongly on RH [40]. Below the critical RH, CU2O formed uniformly on the Cu surface. At the critical RH of 75%, Cu4(S04)(0H)g formed in the region where the (NH4)2S04 particles had been deposited. At 85% RH, a thick corrosion product covered fee entire surface. Particulate contamination such as this is typically believed to be an issue during manufacturing. However, it can also cause field failures in electronic installations, such as telecommunications centers [92]. 

Sulfur dioxide stimulates oxidation. It is the major driving force for corrosion in metropolitan areas. Most of the sulfur acquired by surface is not in the form of gas but as dry deposition. In an urban atmosphere, SO is abundantly found in aerosol particles. Large particles containing ammonia are also found. H2S, SO2 and COS in all these participate directly in the corrosion process. The sulfur compound, COS, hydrolyzes to form H2S and it may form CU2S if the quantity of COS is abundant on the other hand, SO2 may hydrolyze to form a bisulfate ion. 

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