Weight loss corrosion rate measurements

Weight loss corrosion rates, which represent an average of corrosion over the test period, are useless from a predictive point of view, but are often used in conjunction with other measurements for quality assessments. Corrosion kinetics can be measured in different ways. Most favored are electrochemical techniques. They are, however, contrary to common belief, indirect techniques and must be properly calibrated and interpreted to be useful. If corrosion products are soluble in solution (as, for instance, iron carbonate), the buildup of such in solution can be used to monitor how corrosion progresses. Hydrogen, a byproduct of anaerobic corrosion, can also be used to monitor kinetics. Less common, but equally direct, are methods that use the removal of radioactivity from irradiated surfaces. Kinetic measurements have also been carried out with electrical resistance probes. As a general principle, no one method is in itself without some problems and should, therefore, always... 

TABLE 3—Corrosion inhibition measurements in 17 % HCl at 200 F on various steels with three different inhibitors. R = weight loss corrosion rate over integrated LPR measurement. = same for first two hours of exposure, same for 2 to 24 h, ii = same for average of total exposure time. Average 24-h corrosion rates in mpy, P indicates coupon pitted or PP pitted severely. 

Figure 1.69 Schematic diagram showing the variation of cathodic potential with current density for steel in seawater, and the correlation of corrosion rate measured by weight loss. (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

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). 

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. 

Chang and Wei (1990) used electrochemical and conversion-electron Mossbauer spectroscopy (CEMS) methods to study the corrosion behavior of electrodeposited Fe tZnj, wherej = 0.15-0.85,on 1010 steel immersed in a 0.1% NaCl solution at room temperature for 2 weeks. The corrosion rate measured by metal weight loss and electrochemical methods revealed that the Feo,25Zno,75 specimen was more corrosion resistant than the others. CEMS analysis showed that the corrosion product of the pure 1010 steel and the higher iron-containing Fe-Zn alloys on steel in 0.10% NaCl solution is (3-FeOOH. 

FIG. 9—Effect of precorrosion on inhibitor performance. Constant pH kettle test, 55°C, 1 bar CO2,100 ppm inhibitor. Corrosion rates monitored by Fe-counts or weight loss corrected LPR measurements. 

Corrosion in brownstock and post-oxygen washers has been investigated by Bennett and Magar [795] using electrochemical procedures. Their results showed that corrosion is affected by pH, chloride concentration, temperature, and aeration. These tests were complemented with instantaneous corrosion rate measurements by linear polarization resistance methods and by weight loss tests. 

Aqueous corrosion is electrochemical in nature. It is therefore possible to measure corrosion rate by employing electrochemical techniques. Two methods based on electrochemical polarization are available The Tafel extrapolation and linear polarization. Electrochemical methods permit rapid and precise corrosion-rate measurement and may be used to measure corrosion rate in systems that cannot be visually inspected or subject to weight-loss tests. Measurement of the corrosion current while the corrosion potential is varied is possible with the apparatus shown in Figure 1.4. 

Measurement of current flow to/from the coupon and its direction can also be determined, for example, by using a shunt resistor in the bond wire. Importantly, it is also possible to determine corrosion rates from the coupon. Electrical resistance sensors provide an option for in-situ corrosion-rate measurements, as an alternative to weight-loss coupons. 

The combination of immersion, hydrogen collection and weight loss measurement is an easy corrosion evaluation method, particularly for magnesium alloys. The method, first established and used by Song et al. [5] to estimate and monitor the corrosion rate of magnesium in a NaCl solution, has been widely adopted as a common corrosion rate measurement technique for magnesium alloys in various aqueous solutions. The reliability of the method has been theoretically and experimentally demonstrated [24,25] and the details will not be repeated here. 

Variously designed weight-loss coupon, electrochemical and surface analytical techniques have been utilized in REM-based corrosion inhibitors and conversion coatings research. In particular, electrochemical techniques including EIS and polarization measurements have been widely used to evaluate corrosion inhibition by REM compounds under various environmental conditions. Relatively less attention has been paid to the evaluation of localized corrosion inhibition by REM-based compounds, probably because of methodological difficulties and complexities in making accurate localized corrosion rate measurements. Recently developed techniques such as the scanning probe techniques, electrochemical noise analysis and the wire beam electrode are expected to be useful tools in further REM inhibitor research. 

Several measurements can be made after a coupon-type corrosion sensor has been attached to a cathodically protected pipeline. on potentials measured on the coupon are in principle more accurate than those measured on a buried pipe, if a suitable reference electrode is installed in close proximity to the coupon. The potentials recorded with a coupon sensor may still contain a significant IR drop error, but this error is lower than that of surface on potential measurements. Instant-OFF potentials can be measured conveniently by interrupting the coupon bond wire at a test post. Similarly, longer-term depolarization measurements can be performed on the coupon without depolarizing the entire buried structure. Measurement of current flow to or from the coupon and its direction can also be determined, for example, by using a shunt resistor in the bond wire. Importantly, it is also possible to determine corrosion rates from the coupon. Electrical resistance sensors provide an option for in situ corrosion rate measurements as an alternative to weight loss coupons. 

It is significant that most of the data from which a remarkable uniformity of attack is deduced are derived from small isolated panels. This is the most convenient form of specimen for measurements of corrosion rates by loss of weight but it eliminates the important effect of galvanic currents passing between remote parts of a large structure. It is believed that the experience of civil engineers and other users would not support the conclusion suggested by panel tests that corrosion is no faster in tropical than in temperate waters. 

The addition of small amounts of nickel to iron improves its resistance to corrosion in industrial atmospheres due to the formation of a protective layer of corrosion products. Larger additions of nickel, c.g. 36% or 42%, are not quite so beneficial with respect to overall corrosion since the rust formed is powdery, loose and non-protective, leading to a linear rate of attack as measured by weight loss. Figure 3.37 of Pettibone illustrates the results obtained. 

Corrosion rates in normal industrial atmospheres measured as loss of weight over a period are extremely uniform among the various alloys. Table 4.19, last column, gives the corrosion rates (in g m d" ) for a number of alloys determined at Clifton Junction in recent years. The highest value recorded (0-4 g m d ) is equivalent to a rate of penetration of 0-076 mm/y, which is appreciably less than that of mild steel. 

By the use of many commercial abrasive processes, the corrosion resistance of magnesium alloys can be reduced to such an extent that samples of metal that may lie quiescent in salt water for many hours will, after shot blasting, evolve hydrogen vigorously, and the corrosion rate, as measured by loss of weight, will be found to have increased many hundred-fold. The effect in normal atmospheres is naturally much less, yet the activation of the surface is an added hazard and is the opposite of passivation which is essential if later-applied paint finishes are to have proper durability. 

Electrochemical impedance, weight loss, and potentiodyne techniques can be used to determine the corrosion rates of carbon steel and the activities of both sulfate-reducing bacteria and acid-producing bacteria in a water injection field test. A study revealed that the corrosion rates determined by the potentiodyne technique did not correlate with the bacterial activity, but those obtained by electrochemical impedance spectroscopy (EIS) were comparable with the rates obtained by weight loss measurements [545]. 

Corrosion rates are usually expressed as a penetration rate in inches per year, or mills per year (mpy) (where a mill = 10 3 inches). They are also expressed as a weight loss in milligrams per square decimetre per day (mdd). In corrosion testing, the corrosion rate is measured by the reduction in weight of a specimen of known area over a fixed period of time. 

There are a variety of test methods available. Commonly, test specimens consisting of small strips or standard samples of the material are exposed to the process fluid by immersion. The weight loss of the test specimen over a time period at the service temperature is measured in order to determine the corrosion rate. Testing can be carried out on the plant, in the laboratory, or on a pilot plant as demanded by situations. 

ASTM D2688 provides a standard test method for measuring the corrosivity of water via weight loss coupons. And the fact is that unless a suitable protocol such as ASTM D2688 is employed for coupon preparation, installation, and subsequent cleaning, variable rates of corrosion are often obtained. Even when protocols are strictly enforced, some variation is usually found, and therefore it is common practice to employ several coupons at the same time, employing a steel or plastic bypass test rack. 

Figure 3. Carbon corrosion rate versus carbon weight loss for both conventional and graphitized KB-supported Pt catalysts. The carbon corrosion rate (in units of A/g( ) is based on the measured CO2 concentration at the exit of a 50 cnr cell using a GC, assuming 4e /( (T molecule. The carbon weight loss is obtained by integrating the measured CO2 evolution rate over time. The cell is operating on neat H2/N2 (95 °C, 80% RIIjn, and 120 kPaa, s) with potential held at 1.2 Volts versus RHE.

Measurement of mass lost is the conventional method for determining the corrosion rate. The mass loss of an Fe specimen immersed in a corrosion test potential is determined by weighing, (b) Convert the mass loss rate 2.34 x 102 g tf rrT2 into icon using the atomic weight 55.847. (c) What is the difference between the results of the mass loss measurement and the polarization resistance measurement (Numata)... 

In this expression, bd and bc refer to the appropriate anodic and cathodic Tafel constants. Comparison of weight loss data collected as a function of exposure time determined from R , Rf from EIS, and gravimetric measurements of mild steel exposure to 0.5 M H2S04 are often within a factor of two. This suggests that use of Rn in the Stern-Geary equation may be appropriate for the estimation of corrosion rate (147-150). However, Rn measurements may underestimate corrosion rates. / p is often measured at effective frequencies of 1(T2 Hz or less in linear polarization or EIS measurements, while Rn is measured at 1 Hz or greater. An example of this is provided in Fig. 57, which shows the corrosion rate of carbon steel in 3% NaCl solution as a function of exposure time determined by EIS, linear polarization, noise resistance, and direct current measurement with a ZRA. Among these data, the corrosion rates determined by noise resistance are consistently the lowest. 

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