Rate of corrosion

The general rate of corrosion depends on temperature. It is eshmated as follows  

The value of Kt is temperature dependent and applicable for a temperature range of 5 to 150°C. If below 5°C, the temperature is assumed as 5°C for the purpose of calculation. Similarly, if the temperature is above 150°C, it is assumed as 150°C. 

Process engineering and design using Visual Basic 

Mathematically, pH is defined as a negative logarithm of the concentration of hydrogen ions. The concentration of hydrogen ion is calculafed as follows  

Ch+ = concentration of hydrogen ion, molar Co Bicarb = initial ammmt of sodium bicarbonate, molar 

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CO2 corrosion often occurs at points where there is turbulent flow, such as In production tubing, piping and separators. The problem can be reduced it there is little or no water present. The initial rates of corrosion are generally independent of the type of carbon steel, and chrome alloy steels or duplex stainless steels (chrome and nickel alloy) are required to reduce the rate of corrosion. 

Unprotected steel corrodes at a rate which is generally assumed to be 0.1 to 0.2mm per annum. Factors that influence the actual rate of corrosion include the maintenance program applied by the owner - particularly preservation of protective coatings, efficiency of cathodic protection systems in ballast tanks, corrosive properties of the cargo carried and environmental factors such as temperature and humidity. Under extreme conditions it has been known for the annual rate of corrosion on unprotected steel exposed on both surfaces to approach 1mm. 

The electrons are then replaced by the oxidation reaction of Fe to Fe (fonning FeSO if H2SO4 is the acid), and the rate of corrosion is simply the current mduced by metal ions leaving the surface. 

Clearly then, if either water or oxygen are absent, corrosion cannot occur. The presence of an electrolyte, which imparts conductivity to the solution, increases the rate of corrosion. 

Chloride. Chloride is known to significantly increase the rate of corrosion in acidic fluoride media. The level of chloride that can be tolerated in the HF process before corrosion hinders plant operation is quite low. 

Aqueous Corrosion. Several studies have demonstrated that ion implantation may be used to modify either the local or generalized aqueous corrosion behavior of metals and alloys (119,121). In these early studies metallic systems have been doped with suitable elements in order to systematically modify the nature and rate of the anodic and/or cathodic half-ceU reactions which control the rate of corrosion. 

Silver reduces the oxygen evolution potential at the anode, which reduces the rate of corrosion and decreases lead contamination of the cathode. Lead—antimony—silver alloy anodes are used for the production of thin copper foil for use in electronics. Lead—silver (2 wt %), lead—silver (1 wt %)—tin (1 wt %), and lead—antimony (6 wt %)—silver (1—2 wt %) alloys ate used as anodes in cathodic protection of steel pipes and stmctures in fresh, brackish, or seawater. The lead dioxide layer is not only conductive, but also resists decomposition in chloride environments. Silver-free alloys rapidly become passivated and scale badly in seawater. Silver is also added to the positive grids of lead—acid batteries in small amounts (0.005—0.05 wt %) to reduce the rate of corrosion. 

Corrosion by Various Chemicals and Environments. In general, the rate of corrosion of magnesium ia aqueous solutions is strongly iafluenced by the hydrogen ion [12408-02-5] concentration or pH. In this respect, magnesium is considered to be opposite ia character to aluminum. Aluminum is resistant to weak acids but attacked by strong alkaUes, while magnesium is resistant to alkaUes but is attacked by acids that do not promote the formation of iasoluble films. 

The production of hydroxide ions creates a localized high pH at the cathode, approximately 1—2 pH units above bulk water pH. Dissolved oxygen reaches the surface by diffusion, as indicated by the wavy lines in Figure 8. The oxygen reduction reaction controls the rate of corrosion in cooling systems the rate of oxygen diffusion is usually the limiting factor. 

Alloys having varying degrees of corrosion resistance have been developed in response to various environmental needs. At the lower end of the alloying scale are the low alloy steels. These are kon-base alloys containing from 0.5—3.0 wt % Ni, Cr, Mo, or Cu and controlled amounts of P, N, and S. The exact composition varies with the manufacturer. The corrosion resistance of the alloy is based on the protective nature of the surface film, which in turn is based on the physical and chemical properties of the oxide film. As a rule, this alloying reduces the rate of corrosion by 50% over the fkst few years of atmosphere exposure. Low alloy steels have been used outdoors with protection. 

Probably the most serious disadvantage of this method of corrosion study is the assumed average-time weight loss. The corrosion rate could be high initially and then decrease with time (it could fall to zero). In other cases the rate of corrosion might increase veiy gradually with time or it could cycle or be some combination of these things. 

The effect of impurities in either structural material or corrosive material is so marked (while at the same time it may be either accelerating or decelerating) that for rehable results the actual materials which it is proposed to use should be tested and not types of these materials. In other words, it is much more desirable to test the actual plant solution and the actual metal or nonmetal than to rely upon a duphcation of either. Since as little as 0.01 percent of certain organic compounds will reduce the rate of solution of steel in sulfuric acid 99.5 percent and 0.05 percent bismuth in lead will increase the rate of corrosion over 1000 percent under certain conditions, it can be seen how difficult it would be to attempt to duplicate here all the significant constituents. 

Three factors influence the rate of corrosion of metals—moisture, type of pollutant, and temperature. A study by Hudson (1) confirms these three factors. Steel samples were exposed for 1 year at 20 locations throughout the world. Samples at dry or cold locations had the lowest rate of corrosion, samples in the tropics and marine environments were intermediate, and samples in polluted industrial locations had the highest rate of corrosion. Corrosion values at an industrial site in England were 100 times higher than those found in an arid African location. 

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. 

Erosion and Corrosion combined require special consideration. Most of the stainless steels and related corrosion-resistant alloys ow e their surface stability and low rate of corrosion to passive films that develop on the surface either prior to or during exposure to reactive fluids. If conditions change from passive to active, or if the passive film is removed and not promptly reinstated, much higher rates of corrosion may be expected. 

Cavitation corrosion occurs when a surface is exposed to pressure changes and high-velocity flows. Under pressure conditions, bubbles form on the surface. Implosion of the bubbles causes local pressure changes sufficiently large to flake off microscopic portions of metal from the surface. The resulting surface roughness acts to promote further bubble formation, thus increasing the rate of corrosion. 

The corrosion resistance of lead is due to the formation of a thin surface film of an insoluble lead salt that protects the metal from sulfuric acid and related compounds of any strength at ordinary temperatures. Even when (he temperature increases to nearly 100°C the rates of corrosion are still low. However, strong, hot sulfuric acid attacks lead rapidly, especially if the acid is flowing. 

Water was injected into an oil stream using the simple arrangement shown in Figure 9-6. Corrosion occurred near the point shown, and the oil leak caught fire [5]. The rate of corrosion far exceeded the corrosion allowance of 0.05 in. per year. 

Pitting is a form of extreme, localized attack. The rate of corrosion is greater at some areas than at others, resulting in holes in the metal. Heterogenous metal... 

Scale deposits create conditions for concentration-cell corrosion as they do not form uniformly over the metal surface. Sulfate-reducing bacteria thrive under these deposits, producing hydrogen sulfide and, consequently, increasing the rate of corrosion. Due to the following factors, the drilling fluid environment is ideal for scale deposition [189]. These factors are as follows ... 

Figure 4-457. Effect of velocity of flow on the initial rate of corrosion of steel pipe. (From Ref. [197].)...

In the above mechanism, both hydrogen ion and molecule are utilized by SRB to convert SO to H,S. The consumption of hydrogen depolarizes the cathode and leads to an increased rate of corrosion. 

The oil enmeshes in the tail, as shown in Figure 4-480, and provides a mechanical barrier to attack of the aqueous corrodents on the base metal. The oily film also increases the resistance to corrosion current flow and, thus, stifles the rate of corrosion. An advantage of using organic film-forming inhibitors... 

R = rate of corrosion in the absence of an inhibitor R. = rate of corrosion in the presence of an inhibitor... 

Arctic Drilling. Corrosion problems encountered in arctic area drilling are no different from problems faced in other areas of the world. It is a general misconception that during arctic drilling corrosion-related problems are either not very severe or totally absent due to low temperatures. Cool temperatures may slow down the corrosion process. However, they also increase the solubility of oxygen, carbon dioxide and hydrogen sulfide. Therefore, the net result can be an increase in the rate of corrosion. While cold temperatures may cause problems, the temperature fluctuation common in arctic environments can be a more severe source of corrosion-related problems [215]. 

Thus in the system depicted in Figure 53.1, for any metal which is thermodynamically unstable with respect to its dissolved ions in the solution (and this includes most metals of industrial importance) corrosion will occur. The rate of corrosion is determined by several factors ... 

This technique is based upon the detection of corrosion products, in the form of dissolved metal ions, in the process stream. A thin layer of radioactive material is created on the process side of an item of plant. As corrosion occurs, radioactive isotopes of the elements in the construction material of the plant pass into the process stream and are detected. The rate of metal loss is quantified and local rates of corrosion are inferred. This monitoring technique is not yet in widespread use but it has been proven in several industries. 

Verifying that the flow rates of corrosion inhibitor and antifoulanl are adequate for the new operating conditions. 

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