Measuring corrosion rates

Figure 1.62b shows the result of raising the potential of a corroding metal. As the potential is raised above B, the current/potential relationship is defined by the line BD, the continuation of the local cell anodic polarisation curve, AB. The corrosion rate of an anodically polarised metal can very seldom be related quantitatively by Faraday s law to the external current flowing, Instead, the measured corrosion rate will usually exceed... 

Danielson, M. J., Analysis of Errors in Using The Two Electrode and Three Electrode Polarisation Resistance Methods In Measuring Corrosion Rates , Corrosion, 36, No. 4, 174-178, April (1980)... 

Some data from corrosion-monitoring probes do not measure corrosion rate, but rather give other useful information about the system. For example, suppose conditions change dramatically during a process upset. An ejq)erienced corrosion engineer can examine the data and correlate it with the upset conditions. Such analysis can provide insight into the process and help to improve performance and extend equipment lifetime. Changes in simple parameters such as pH, ion content, and temperature may lead to detection of a process upset. Without careful analysis, process upsets can reduce the corrosion lifetime of equipment and even cause a system failure. 

As the impedance and harmonic analysis techniques gave different types of data from each other, a direct comparison between Tables 1 and 2 is difficult. However, it should be anoted that G4 gave both semicircular behaviour and a reasonably high corrosion rate. This similarity is also true for A14 and A15. All and A13 showed smaller but still measurable corrosion rates, together with semicircular impedance behaviour and the absolute values measured varied similarly as before. 

This is an electronic method used to measure corrosion rate in mils per year. The corrosion probe can be inserted through a packing gland. It is read periodically with a portable instrument that measures the change in electrical conductivity of the probe. It is simple, but perhaps a little less reliable than the coupon. 

Similarly, a failure to consider in advance the monitoring techniques to be used, or the real relevance of the measured corrosion rate to be thus reported, renders the practice virtually meaningless and doomed to failure. 

Electrochemical techniques alone cannot reveal all the relevant aspects of chromate inhibition, and key characterization experiments involving surface analysis, solution analysis, or other techniques are required to help understand inhibition. For this reason, several useful nonelectrochemical techniques are also discussed. These techniques provide a means for examining the effects of inhibition under free corrosion conditions where electrochemical methods are not well suited for measuring corrosion rate. 

The methods of measuring corrosion rates in the course of testing corrosion inhibitors are conventional weight loss, electrochemical techniques such as linear polarization resistance, potentiodynamic polarization, AC impedance, and electrochemical potential or current noise. 

Corrosion of the glass-making melters must be maintained at an absolute minimum to increase the lifespan of the melter. Laboratory-measured corrosion rates indicate that melter lifetimes of several years can be achieved with high chrome oxide or zircon refractories metallic melters may have lifetimes of several months if alloys such as Inconel 690 are used. These conclusions have been reached on the basis of extrapolation of laboratory tests. Long-term tests, particularly with waste glasses in engineering-scale continuous melters, have not yet been made. 

Thermodynamic predictions were consistent with experimentally measured corrosion rates and open circuit potentials. The results indicate enhanced corrosion of stainless alloys containing chromium may be expected in supercritical water. These corrosion rates appear comparable to those for mild steel or iron. 

D.A. Jones, Polarization Methods to Measure Corrosion Rate, Principles and Prevention of Corrosion, Macmillan Publishing Co., 1992, p 142-166... 

Weight loss has been measured for more than 40 modem iron coupons each exposed for approximately 2 years in the waterlogged peat and gyttja layers in Nydam. The results have demonstrated a close correlation between archaeological excavations in the area and measured corrosion rates (Figure 9). 

Both the H2S concentration over the range of 0-1.0 v/o of the CGA gas and the temperature controlled the measured corrosion rates. Figure 1 illustrates the effect of temperature on corrosion rates of several alloys and coatings in the CGA gas containing 1 v/o H2S. Alloys AISI 309, AISI 310, and IN-800 demonstrate a clear temperature dependence of total oxidation-corrosion in 1000 hr. The 309 alloy had a scatter band of 5 to 125 mils total metal loss for four specimens at 1650°F. This is typical of borderline alloys that undergo time-dependent transitions to accelerated corrosion rates. Total corrosion of aluminized 310 and 800 was relatively unaffected by temperature over the range of 1500°-1800°F for 1000 hr exposures. 

Before heginning a series of experiments to measure corrosion rates or to study some other aspect of corrosion, it is important to determine clearly what the goal of the work is. Usually, such studies require the use of an accelerated test method to provide answers to problems quickly. Exposure testing in the service environment is an important tool in corrosion engineering, hut the time frame for exposure tests is often too long for timely decisions on design. 

The linear polarization technique estimates instantaneous corrosion rates under various process conditions. The corrosion current, according to the Stem-Geary equation, is inversely proportional to polarization resistance, which allows the measured polarization resistance to be normalized directly into corrosion rates. Because the current follows the appHed overvoltage, the polarization resistance curve is plotted automatically. Because this technique accurately measures corrosion rates <0.1 mpy, it is of a great importance in water distribution systems and food industries that face problems with traces of impurities and contamination. It can be used to measure the corrosion rates in civil engineering structures that cannot be subjected to weight loss measurements. Usually, Hnear polarization measurements are executed in 10 min. As shown in Fig. 5.3, the current as a... 

R. Bandy, D. A. Jones, Analysis of errors in measuring corrosion rates by linear polarization, Corrosion 32 (1976) 126-134. 

Atmospheric corrosion studies were performed in the Spanish Canary Islands exposing zinc, copper, and carbon steel sheets to the subtropical and coastal environment for a period of 3 years [45-47]. The first part of this study was to determine the corrosion rates on each sample at different times. The study also performed an analysis of the atmospheric conditions at the thirty-five test sites on the islands. With this information, the atmospheres were classified according to ISO. Measured corrosion rates were compared with the expected ISO values. The corrosion rates obtained based on ISO were determined deficient throughout the studied region... 

An example of measured corrosion rates on steel in seawater is shown in Figure 6.8. If this diagram is redrawn with v as the variable along the horizontal axis, we will get an S-shaped curved in this case also, as we have in Figure 6.6. 

The de Waard-Milliams equation is often presented graphically in a nomogram, as shown in Figure 6.14. Here a scale for a deposit factor (scale factor) is also included. This scale factor scale has been determined by comparison of measured corrosion rates at various temperatures above 60°C with those calculated from Equation (6.18). The read corrosion rate at a certain combination of CO2 pressure and temperature has to be multiplied by the read deposit (scale) factor. 

In marine atmospheres measured corrosion rates are related to the rates of chloride deposition. 

Electric current flows in the soil from the buried anode to the underground structure to be protected. Therefore, the anode must be connected to the positive pole of the rectifier, and the structure to the negative pole. All cables from the rectifier to the anode and to the structure must be electrically insulated. If not, those from the rectifier to the anode will act as an anode and deteriorate rapidly, while those from the rectifier to the structure may pick up some of the current, which would then be lost for protection. The specific metal and environment will determine the current density required for complete protection. The applied current density must always exceed the current density equivalent to the measured corrosion rate under the same conditions. Therefore, as the corrosion rate increases, the impressed current density must be increased to provide protection. 

It is also very difficult to measure corrosion rates in these situations as they are usually underwater or under waterlogged conditions such as a failed membrane. 

By measuring corrosion rates over a period of time we can estimate the time to cracking and knowing the distribution of corrosion rates, cover depths, etc. a cracking rate can be established by adding the time to cracking to the initiation time. An empirical condition curve can be calculated and the time taken to reach an unacceptable level can be determined. 

The current density required for complete protection depends on the metal and on the environment. It can be seen from Fig. 5.15 in Section 5.11 that the applied current density must always exceed the current density equivalent to the measured corrosion rate in the same environment. Hence, the greater the corrosion rate, the higher must be the impressed current density for protection. 

The environmental factors that tend to accelerate metal loss include high humidity, high temperature and proximity to the ocean, extended periods of wetness and the presence of pollutants in the atmosphere. The small amount of carbon dioxide normally present in the air neither initiates nor accelerates corrosion. Vernon [57] was the first to study the corrosion rate of steel coupons in the presence of well-defined atmospheres. Atmospheric gases such as CO2, SO2, NO2, HCl, etc. after getting dissolved in the moisture layer on the metal surface, these gases result in a number of ions and ionic species like H", CF, COa , NOa , S04 , etc. They measured corrosion rate was as a function of time, relative humidity and... 

Where, W is weight loss (mg), A is area of the specimen (cm ), D is density of the specimen (gm/cm ), T is exposure time (hours) and unit pm/year is micro-metre/year. Indirect methods of corrosion rate measurement involve anodic/ cathodic reaction, consideration of current potential relationship or polarisation resistance values. Tafel extrapolation method is the most popular laboratory methods for measuring corrosion rate of a metal from electrochemical data in a corrosive medium. 

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