Atmospheric corrosion chloride

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

There are many special factors controlling atmospheric bimetallic corrosion that entitle it to separate treatment. The electrolyte in atmospheric corrosion consists of a thin condensed film of moisture containing any soluble contaminants in the atmosphere such as acid fumes from industrial atmospheres and chlorides from marine atmospheres. This type of electrolyte has two characteristics which are summarised in a paper by Rosenfel d . 

Sulphur oxides These (SO2 is the most frequently encountered oxide) are powerful stimulators of atmospheric corrosion, and for steel and particularly zinc the correlation between the level of SO2 pollution and corrosion rates is good However, in severe marine environments, notably in the case of zinc, the chloride contamination may have a higher correlation coefficient than SO2. 

In marine atmospheres magnesium chloride is formed and eventually oxychloride by reaction with magnesium hydroxide formed at the same time. Since the chloride is hygroscopic, moisture is attracted and the corrosive effect is hence much worse than that of water alone. 

The relative susceptibility of metals to atmospheric corrosion varies widely with the type of contaminant, e.g. zinc and cadmium, two metals that are used for the protection of steel in exposed environments, are both rapidly attacked by organic acidson the other hand, aluminium alloys resist attack by organic acids but may be rapidly corroded by chlorides, especially at crevices or areas of contact. 

It confirms that the acceleration rate caused by chloride ions on atmospheric corrosion of steel and copper depends on the characteristics of rain regime. For a place having high amount and time of rain, a lower acceleration on corrosion rate should be expected for a given chloride deposition rate... 

The adsorption isotherms for metallic surfaces are reported in the literature however, an important part of the atmospheric corrosion process takes place under rust layers, which play a decisive role in the long-term course of corrosion because of its sorption capacity for water. The influence of the chloride and sulfate anions has a real effect only when the corrosion products layer is already formed. Thus, the adsorption isotherms of the steel corrosion products formed in different atmospheres were determined. 

The four important areas of application of carbon steels are (i) atmospheric corrosion (ii) corrosion in fresh water (iii) corrosion in seawater and (iv) corrosion in soils. The atmospheric corrosion of steel is caused by major environmental factors such as (i) time of wetness as defined by ISO 9223-1992 (ii) sulfur dioxide in the atmosphere due to the combustion of fossil fuels and (iii) chloride carried by the wind from sea. The equations for corrosion rates of carbon steel by multiple regression analysis have been obtained.1... 

Atmospheric corrosion of lead involves exposure to industrial, rural and marine environments. The mode of corrosion in the three environments is different. The rural environment consists of humidity, airflow and rainfall, which may be considered to be innocuous. The marine environment consists of chloride entrained in air and could... 

Corrosion products such as the oxides, hydroxides, carbonates, sulfates, basic sulfates hydroxy carbonates, hydroxy chlorides are formed in the various environments (marine, urban, rural, industrial) and the initially loosely bound products may become adherent in the course of time. Corrosion can occur in the pores of the corrosion product layers. The low corrosion rate observed in atmospheric corrosion, R has been expressed as ... 

Atmospheric Atmospheric corrosion due to the combined effects of rain and the deposition of salt and other pollutants will affect most equipment. Corrosion occurs while the metal surface is wet, and is strongly influenced by the composition of deposits (such as sulfates from industrial atmospheres and chlorides from marine atmospheres). External corrosion of steel and stainless steel process equipment beneath thermal insulation and fireproofing is of particular concern. 

Aerosol particles are chemical mixtures of a number of ionic species, including ammonium, sulfate, chloride, nitrate, and hydrogen. The actual chemical mixture of each particle reflects the chemical conditions in which the particle was formed. Once adsorbed on a metal surface, the water-soluble part of each particle acquires water from the atmosphere and deliquesces, whereby it transforms into a concentrated aqueous solution. The ionic constituents that are liberated into the aqueous film may have a significant influence on the atmospheric corrosion processes. Moreover, many particles possess hygroscopic properties and retain water. Lienee, a typical particle may triple its volume when the relative humidity increases from dry to 90%. 

Atmospheric corrosion of metals is differentiated from the other forms of corrosion due to exposure of metals to different atmospheres rather than immersion in electrolytes. The spontaneous atmospheric corrosion of materials is controlled by the temperature, the relative humidity, the time of wetness, the pH of the electrolyte, and the presence of contaminants such as chlorides, NH3, SO2, NO2, and acidic fogs. In most cases, the rate equations have hmited validity due to different local atmospheric conditions. Metals spontaneously form a solid metal oxide film when exposed to dry atmospheres. The barrier oxide film reaches a maximum thickness of 2-5 nm [1-6]. The corrosion rate of metals exposed to a wet atmosphere is similar to that observed during immenion in aerated water in the presence of dissolved oxygen. Atmospheric corrosion rates decrease in dry atmospheres with corrosion mechanisms that are different from those in wet atmospheres. 

The pollutants include SO2, nitrogen oxides, chlorides, and phosphates. All gases in the Troposphere (Ne, Kr, He, and Xe) do not participate in atmospheric corrosion [11]. Only oxygen acts as an oxidizer in a cathodic reaction. The presence of CO2 in the electrolyte ( 300 ppm) decreases pH and increases the corrosion rate of metals. 

Atmospheric corrosion of metals is affected by contaminants such as chlorides from deicing or from airborne salt in marine atmospheres. Other sources are HCl production from the burning of chlorine-containing coals and the emission of chlorine from industrial processes. Hydrochloric acid is formed by photodissociation of chlorine, resulting in the production of chlorine radicals that are hydrogenated with organic compounds. 

The model for atmospheric corrosion tmder high chloride concentration su ested by Kamimura et al. [32] is based on the separation of cathode and anode sites under the rust and the thin electrolyte. The pH at the anode compartment is affected by the chloride ion concentration and decreased to 1.5 by the hydrolysis of ferric ions and the formation of P-FeOOH. Chloride ions accumrrlate at the anode site and initiate the oxidation of ferrous ions to ferric ions. Accumulated chloride ions increase ferric ion solubility in the electrolyte and accelerate the hydrolysis of ferric ions, causing the pH at the anode to decrease. Low pH at the metal-electrolyte interface accelerated the formation of P-FeOOH. The atmospheric corrosion process is summarized as follows ... 

Atmospheric corrosion of steel in the presence of chloride is summarized in Fig. 10.6 [32]. 

In wet atmospheres, nickel initially forms NiO and (NiOH)2 [35,36]. Nickel sulfates are present as corrosion products on the surface in outdoor exposures [37].Jouenefatmospheric corrosion of nickel in industrial, urban, and rural atmospheres. Nickel corrodes through a pitting corrosion process. The highest corrosion rates were observed in industrial areas. The corrosion products were mainly sulfates, chlorides, and n ligible amounts of nitrates surrounded by carbonate species. The pitting corrosion process occurs in two steps on nickel surfaces exposed to an outdoor atmosphere, as shown in Fig. 10.9 [38]. 

The International Standard Organization (ISO) developed a corrosivity classification system verified through exposure that has been carried worldwide. The ISO classification system is based on the assumptions that only the time of wetness and the concentration of pollutants in the atmosphere, SO2 and chlorides, control the corrosion rates of metals. Table 10.1 shows the Hst of ISO standards related to atmospheric corrosion of metals [39]. 

The EIS technique is effective for studying the atmospheric corrosion of metals in the range of 5-100% RH in the presence of chlorides [50], This information is potentially... 

Experimental measurements indicated that the change in the thickness of the electrolyte affects the mass transport of oxygen, hydration ofdissolved metal ions, and accumulation of corrosion products [1,54—56]. Dubuisson et al. [57] investigated the atmospheric corrosion of galvanized steel in a micrometric electrolytic droplet containing sulfate and chloride. The measurements were performed in an electrochemical microcell through controlled... 

T. Kamimura, K. Kashima, K. Sugae, H. Miyuki, T. Kudo, The role of chloride ion on the atmospheric corrosion of steel and corrosion resistance of Sn-bearing steel, Corros. Sci. 62 (2012) 34—41. 

As mentioned in Chapter 3, the most significant deviations from the theoretical Pourbaix diagram in practice are that chloride or other aggressive species may destroy the passivity and that impurities in the metal may cause weak points in the oxide, with pitting as a possible consequence (Section 7.6). For pure aluminium, the corrosion resistance decreases considerably when the content of impurities in the metal increases from 0.01 to 1%. However, even 99% aluminium resists neutral atmospheres and chloride-free water very well. In seawater, pitting will usually occur, but the weight loss is low and the pits shallow (Section 7.6). 

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