Corrosion calculation

Vincentini, B., Sinigaglia, D. and Taccani, G., Crevice Corrosion Calculation of the Voltage and Current Distribution Along the Crevice , Werks. Korros., 22, 916 (1971)... 

Cobalt (Co) is a metal that is added to steel to improve its resistance to corrosion. Calculate both the number of moles in a sample of cobalt containing 5.00 X 10 ° atoms and the mass of the sample. 

Rate of corrosion calculated from I- and 4-year tests, which showed a linear rate of corrosion with time. 

The exhaust steam from the turbine of a power plant, at 0.3 bar and a quality of 5 %, is condensed to saturated liquid with cooling water entering at 25 C. If the cooling water leaves at 50 C (higher temperatures are undesirable to avoid excessive corrosion), calculate its exergy per kg of steam condensed. 

The war itself also drove the development of improved and miniaturised electronic components for creating oscillators and amplifiers and, ultimately, semiconductors, which made practical the electronic systems needed in portable eddy current test instruments. The refinement of those systems continues to the present day and advances continue to be triggered by performance improvements of components and systems. In the same way that today s pocket calculator outperforms the large, hot room full of intercormected thermionic valves that I first saw in the 50 s, so it is with eddy current instrumentation. Today s handheld eddy current inspection instrument is a powerful tool which has the capability needed in a crack detector, corrosion detector, metal sorter, conductivity meter, coating thickness meter and so on. 

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

Corrosion occurs even if the two reactants involved are not at standard conditions. In this case the nonstandard equiUbrium potential for each reaction, often referred to as the reversible potential, can be calculated from the Nemst equation. Additional information on thermodynamic aspects of corrosion can be found in Reference 10. 

One of the principal reasons for failure due to reaction with the service environment is the relatively complex nature of the reactions involved. Y"et, in spite of all the complex corrosion jargon, whether a metal corrodes depends on the simple elec trochemical cell set up by the environment. This might give the erroneous impression that it is possible to calculate such things as the corrosion rate of a car fender in the spring mush of salted city streets. Dr. M. Pourbaix has done some excellent work in the application of thermodynamics to corrosion, but this cannot yet be applied direc tly to the average complex situation. 

If it is assumed that localized or internal corrosion is not present or is recorded separately in the report, the corrosion rate or penetration can be calculated alternatively as... 

The primaiy use of this laboratoiy technique today is as a quick check to determine the order of magnitude of a corrosion reaction. Sometimes the calculated rate from an immersion test does not look correct when compared to the visual appearance of the metal coupon. 

The CBD diagram can provide various lands of information about the performance of an aUoy/medium system. The technique can be used for a direc t calculation of the corrosion rate as well as for indicating the conditions of passivity and tendency of the metal to suffer local pitting and crevice attack. 

Corrosion Rate by CBD Somewhat similarly to the Tafel extrapolation method, the corrosion rate is found by intersecting the extrapolation of the linear poi tion of the second cathodic curve with the equihbrium stable corrosion potential. The intersection corrosion current is converted to a corrosion rate (mils penetration per year [mpy], 0.001 in/y) by use of a conversion factor (based upon Faraday s law, the electrochemical equivalent of the metal, its valence and gram atomic weight). For 13 alloys, this conversion factor ranges from 0.42 for nickel to 0.67 for Hastelloy B or C. For a qmck determination, 0.5 is used for most Fe, Cr, Ni, Mo, and Co alloy studies. Generally, the accuracy of the corrosion rate calculation is dependent upon the degree of linearity of the second cathodic curve when it is less than... 

Electrochemical Measurement of Corrosion Rate There is a link between elec trochemical parameters and actual corrosion rates. Probes have been specifically designed to yield signals that will provide this information. LPR, ER, and EIS probes can give corrosion rates direc tly from electrochemical measurements. ASTM G102, Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, tells how to obtain corrosion rates directly. Background on the approximations made in making use of the electrochemical measurements has been outlined by several authors. 

Proper economic analysis will allow comparison of alternatives on a sound basis. Detailed calculations are beyond the scope of this section. The reader should review the material in the NACE Publication 3CI94, Item No. 24182, "Economics of Corrosion, Sept., 1994. 

Minimum exposure time is calculated as number of days corrosion rate is in milli-inches per year. 

In the application of magnesium anodes for enamelled boilers, the consumption rate of the anodes is determined less by current supply than by self-corrosion. The calculation of life from data on protection current requirement, /, and anode mass, m, is difficult because the a value is so low. 

In the Verband Deutscher Elektrotechniker (VDE) regulations [1,4], no demands are made on the accuracy of the measured or calculated voltage drops in a rail network. An inaccuracy of 10% and, in difficult cases, up to 20%, should be permitted. A calculation of the annual mean values is required. If the necessary equipment is not available, a calculation is permitted over a shorter period (e.g., an average day). Voltage drops in the rail network only indicate the trend of the interference of buried installations. Assessment of the risk of corrosion of an installation can only be made by measuring the object/soil potential. A change in potential of 0.1 V can be taken as an indication of an inadmissible corrosion risk [5]. 

A large number of parameters are involved in the choice of the corrosion protection system and the provision of the proteetion eurrent these are deseribed elsewhere (see Chapters 6 and 17). In partieular, for new locations of fixed production platforms, a knowledge of, for example, water temperature, oxygen content, conductivity, flow rate, chemical composition, biological activity, and abrasion by sand is useful. Measurements must be carried out at the sea location over a long period, so that an increased margin of safety can be calculated. 

Nodal points of the platform require special attention for corrosion protection. Therefore the anodes have to be installed in the vicinity of these points, as indicated in Fig. 16-4. The spacing must be sufficiently large that the welded Joints of the nodes do not lie in the area of the lap Joints. The effort for calculating the optimal distribution with the lowest weight of anodes is considerable and has led to computer programs by which the anode distribution can be estimated [11]. 

Ten percent of the anode mass is calculated for aluminum ships. The anode supports must also be of aluminum in order to allow them to be welded and to avoid bimetallic corrosion. 

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