Data for corrosion

Figure 3.53 has been derived from data for corrosion by nitric acid solutions of an alloy nominally containing 29% chromium and 0-8% carbon. It... 

It is likely that combined CMT and EC measurements can contribute to further information about the basic details of several other corrosion processes and provide more reliable data for corrosion rates than EC measurements alone. Concerning the possible use of CMT measurements as a routine test of technical products, some of the limitations (it can t be used with iron and steel in aerated solutions or with other metals in bicarbonate solutions) are rather serious. However, there are still many applications where the fast and quantitative results obtained by CMT measurements would seem advantageous. 

The experimental arrangement for potentiodynamic polarization experiment is shown in Figure 1.26. The experiment is done using the software, and polarization curves (both anodic and cathodic branches of polarization) are recorded at a suitable scan rate. The software performs the calculations and gives the data for corrosion potential and corrosion current density for the system on hand. 

F. Mansfeld, Recording and Analysis of AC Impedance Data for Corrosion Studies, I. Background and Methods of Analysis, Corrosion, Vol 36 (No. 5), 1981, p 301... 

F. Mansfeld, M.W. Kending, T. Tsai, Recording and analysis of AC impedance data for corrosion studies II. Experimental approach and results. Corrosion 38 (1982) 570—579. 

Models with simplified geometries embodying descriptions of corrosion phenomena based on first principles, as well as existing measured and calculated data for corrosion parameters, are commercially available. Some codes incorporate mixed potential models that are used for the prediction of corrosion potential and current density. Fundamental concepts are used but calibration with experimental data is frequently required in order to estimate values for poorly known model parameters. 

Mansfeld, F., Kendig, M., and Tsai, M. W., "Recording and Analysis of AC Impedance Data for Corrosion Studies, Part n. Experimental Approach and Results, Materials Performance, Vol. 38, No. 11, November 1982, p. 510. 

The automotive industry has a huge amount of data for corrosion in various service environments. The VICT has a promising correlation to field data one criticism that is sometimes brought against this test is that it may tend to produce filiform corrosion at a scribe. 

It is customary to use the lowest boundary (10" ) as a practical indication of the corrosion stability of a metal and its solid products (Figure 4.14). The usefulness of this graphical representation of thermodynamic data for corrosion studies was discussed by Pourbaix who showed three possible states of a metallic material [3] ... 

A detailed example of cost data for corrosion problems affecting the various broad sectors of the electric power industry has recently been published for the United States [15 16]. The compiled and sorted costs are presented in Table 8.10 with the highest cost listed first. These dollar amounts include corrosion-related costs associated with both operations and maintenance and depreciation. They also include both direct and indirect corrosion-related costs. 

Design parameters as a function of temperature and design temperature limits are set forth in the ANSI/ASME B31 Piping Codes for a very broad range of materials. These codes, and the additional information available from manufacturers, vendors, and technical societies such as the National Association of Corrosion Engineers provide ample data for the selection of materials for piping systems (1—13). 

Materials of Construction. Resistance of alloys to concentrated sulfuric acid corrosion iacreases with increasing chromium, molybdenum, copper, and siUcon content. The corrosiveness of sulfuric acid solutions is highly dependent on concentration, temperature, acid velocity, and acid impurities. An excellent summary is available (114). Good general discussions of materials of constmction used ia modem sulfuric acid plants may be found ia References 115 and 116. More detailed discussions are also available (117—121). For nickel-containing alloys Reference 122 is appropriate. An excellent compilation of the relatively scarce Hterature data on corrosion of alloys ia Hquid sulfur trioxide and oleum may be found ia Reference 122. 

Table 9. Corrosion Data for ASTM Grade-2 Titanium ...

Each reactant and product appears in the Nemst equation raised to its stoichiometric power. Thermodynamic data for cell potentials have been compiled and graphed (3) as a function of pH. Such graphs are known as Pourbaix diagrams, and are valuable for the study of corrosion, electro deposition, and other phenomena in aqueous solutions.Erom the above thermodynamic analysis, the cell potential can be related to the Gibbs energy change... 

Deflagration Arrester Testing For end-of-line and tank vent flame arresters, approval agencies may require manufacturers to provide users with data for flow capacity at operating pressures, proof of success during an endurance burn or continuous flame test, evidence of flashback test results (for end-of-hne arresters) or explosion test results (for in-line or tank vent arrester applications), hydrauhc pressure test results, and results of a corrosion test. 

In the absence of factual corrosion information for a particular set of fluid conditions, a reasonably good selection would be possible from data based on the resistance of materials to a very simifar environment. These data, however, should be used with some reservations. Good practice calls for applying such data for preliminary screening. Materials selected thereby would reqmre further study in the fluid system under consideration. 

As to the effec t of time, there is no universal law that governs the reaction for all metals. Some corrosion rates remain constant with time over wide ranges, others slow down with time, and some alloys have increased corrosion rates with respect to time. Situations in which the corrosion rate follows a combination of these paths can develop. Therefore, extrapolation of corrosion data and corrosion rates should be done with utmost caution. 

For example, consider localized corrosion. Although data from corrosion probes indicate corrosion rate, it is not possible to tell that localized corrosion is the problem. 

When a clean steel coupon is placed in oxygenated water, a rust layer will form quickly. Corrosion rates are initially high and decrease rapidly while the rust layer is forming. Once the oxide forms, rusting slows and the accumulated oxide retards diffusion. Thus, Reaction 5.2 slows. Eventually, nearly steady-state corrosion is achieved (Fig. 5.2). Hence, a minimum exposure period, empirically determined by the following equation, must be satisfied to obtain consistent corrosion-rate data for coupons exposed in cooling water systems (Figs. 5.2 and 5.3) ... 

Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

The data for the average decrease in metal thickness in 4 years and the linear corrosion rate are given in Table 4-2. In addition, extrapolations of the rate for 50 and 100 years are given, which are of interest for the corrosion likelihood of objects buried in earth. It can be seen from the results that film formation occurs in class I soil. In class II soils, the corrosion rate decreases with time only slightly. In class III soils, the decrease with time is still fairly insignificant. 

Vanadium-Sodium Compounds Most Corrosive. Physical property data for vanadates, phase diagrams, laboratory experiments, and numerous field investigations have shown that the sodium vanadates are the lowest melting compounds and are the most corrosive to metals and refractories. These compounds are thought to form by either the vapor phase reaction of NaCI and V2O5 or by the combination of fine droplets of these materials upon the cooler parts of combustion equipment. 

Many fluorides are corrosive to glass and some metals Even the very corrosive anhydrous fluorides, however, can be handled at room temperature in steel, stainless steel, copper, and Teflon equipment For reactions at higher temperature, nickel and Monel metal are indispensable More data on corrosion of matenals are given in pages 22 and 23 of Chemistry of Organic Fluorine Cowjpoundi, published in 1676... 

Owing to the laborious task of obtaining corrosion rates from gravimetric measurements, data for the effect of exposure time on corrosion rates have been very limited. However, with the more recent use of polarisation resistance measurements it would appear that in the absence of macro-biofouling... 

Little scientific examination of the deterioration of materials at depth has been undertaken except that by the US Naval Civil Engineering Laboratory and Naval Research, Laboratory. The results of this work were reported by Reinhart in 1966 and more recently the work has been reviewed by Kirk . Typical corrosion data for a selection of metals exposed in the Pacific Ocean at several sites and for different times are shown in Table 2.19 and are compared with results obtained in surface waters at Wrightsville Beach by International Nickel Inc. 

Manufacturers and specialist materials development associations publish extensive corrosion data in the form of monographs, and this form of presentation is also used in national standards. The most recent comprehensive text in this category is perhaps the publication by the Zinc Development Association . The work is important in that the section on chemicals also deals with common, though complex, chemical formulations, e.g. Are-extinguisher fluids, soaps and syndets, agricultural chemicals such as pesticides and fertilisers. This publication also demonstrates the mammoth task of recording all the available data for just one material. A comparable book for mild steel would probably be much larger, whereas for many other materials the information has not yet been determined. Thus at best, only very incomplete data are available in this form. 

Phosphoric acid The austenitic grades are resistant to all strengths up to 80°C but are limited to 30-40% concentration at boiling point, the molybdenum-bearing types having the best corrosion resistance. Some test data for various types is shown in Fig. 3.28. Industrially, this acid is often encountered in an impure state with appreciable amounts of sulphuric and hydrofluoric acid present so that process testing is likely to be particularly necessary. The super-austenitic steels have enhanced resistance to phosphoric acid solutions. 

In sea-water flowing at slower velocities and more especially in stagnant conditions, pitting and crevice corrosion may develop, particularly beneath deposits and marine growths at the surface of the metal. Some data for the Ni-30 Cu Alloy 400 are shown in Fig. 4.40 the corrosion was mostly pitting. 

Table 7.31 lists data for the corrosion of nickel and some nickel alloys by ammonia at 500°C. At higher temperatures the more complex hardened nickel-chromium-base alloys are more resistant than the binary alloys and... 

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