Carbonic acid iron oxide corrosion

Oxygen corrosion in condensate pipelines is recognizable as large pits and are a typical result. Where the condensate pH level is low (say, due to the presence of carbonic acid), the pits may be particularly large, but as the pH level rises (say, due to the use of neutralizing amines), the layer of iron oxide corrosion product becomes more protective and the resulting pits tend to be smaller. 

Partial blocking effect was first identified for pure iron in contact with aerated sulphuric acid medium [55]. Corrosion of carbon steel in sodium chloride media clearly showed the porous layer effect (see Section 5.2) [74]. The same effect was found for zinc corrosion in sodium sulphate [75] and the properties of the layer which was demonstrated to be formed of an oxide/hydroxide mixture were further used for building a general kinetic model of anodic dissolution [76], usable for measurement of the corrosion rate from impedance data. 

From the above equation we see that the air oxidation of iron is favored in acidic solution lower pH values would have more positive Eam- Increased levels of carbon dioxide and water generate carbonic acid that lowers the pH of the medium, which makes the corrosion process more favorable. 

The three simultaneously necessary conditions for electrolytic corrosion to occur are (1) an electrolyte, which is usually water containing dissolved salts (2) a corrosion cell is composed of an anode, the area being corroded (i.e., oxidized), and a cathode, the area where electrons enter the electrolyte and (3) the anode and cathode are connected by an electronically conductive (i.e., metallic) path. Figure 21.10 illustrates corrosion of iron exposed to wet CO2 (carbonic acid). 

The chemical components of calcium carbonate — dissolved calcium ions and carbon dioxide — are widely distributed. Calcium is the fifth most common element in the earth s crust (after oxygen, silicon, aluminium and iron). It was extracted from early igneous rocks by the combined effects of erosion by the weather and corrosion by acidic gases (oxides of sulfur, oxides of nitrogen and carbon dioxide dissolved in rain water). Carbon dioxide makes up about 0.03 % by volume of the earth s atmosphere and is dissolved in both fresh and sea water. Combination of dissolved calcium ions and carbon dioxide resulted in the sedimentary deposition of calcium carbonate, which was subsequently converted into limestone rock. Early limestones (Precambrian — Table 2.1) are believed to have been deposited as precipitates of CaCOa, and/or as a result of the biochemical activity of very simple organisms, such as bacteria. 

Corrosion is an electrolytic process that requires an anode, a cathode, and an electrolyte. In reinforced concrete the reinforcing steel becomes covered in a passivating layer of y-iron oxide formed in the high-pH environment ( pH 13) provided by the Portland cement. Unfortunately, the high pH can be neutralized by acidic gases such as carbon dioxide in the atmosphere. Once the pH in the concrete drops below 10, generahzed corrosion of the steel will take place in the low-pH area. 

Although atmospheric corrosion of ferrous metals is related first to the amount of moisture in the air (relative humidity), atmospheric moisture alone has no influence on the corrosion of stainless steel. Of primary importance are the effects of such atmospheric contaminants as particulate chlorides and iron-based dust. Sulfur-based acids will promote corrosion while oxides of nitrogen improve the resistance. Contaminants such as hydrocarbons, ammonia, and oxides of carbon have no effect. 

Atmospheric corrosion of steel is a function of location. In country air the products of corrosion are either oxides or carbonates in industrial atmospheres sulfuric acid is present producing iron sulfate and near the ocean some salt is in the air producing iron chlorides. Corrosion takes place more rapidly in industrial areas because of the presence of the acid, and it is higher both near cities and near the ocean because of the higher electrical conductivity of the rain and the tendency to form soluble chlorides or sulfates, which cause the removal of protective scale. 

My mother still thiirks I m very smart. Forty-eight years ago, I removed rust stains from our bathtub with Coca-Cola. It is the carbonic acid in Coke that removed the iron oxide rust stains from the tub. This is an illustration of the corrosive nature of carbonic acid brought into contact with ferric compounds. The carbotric acid is formed from CO2 dissolved in water. 

HCl is also applicable for removing certain scales, such as calcium carbonate, iron carbonate, iron oxides, and iron sulfide. Organic acids, such as acetic acid and formic acid, are sometimes used in place of HCl, especially in high-temperature applications, where HCl corrosion can be severe. 

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