Uniform corrosion mass loss tests

The simplest uniform corrosion tests are those based on coupon immersion and mass loss tests. The tests provide data on the uniform metallic corrosion rate most often expressed in English units as mils per year (mpy) or in metric units as millimeters per year (mm/y). While there are various standards for such tests, the predominant methods are those prescribed by ASTM G 1, "Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens. This standard contains a comprehensive reference list of other ASTM standards that also apply, depending on the type of tests of interest to the investigator. 

Electrochemical tests are often preferred to mass loss studies as a method of measuring uniform corrosion. Such tests are preferred because in theory they (1) provide a realtime measurement of the metaUic corrosion rate, (2) can provide time-corrosion rate data on a single coupon, and (3) are rapid to perform. Disadvantages of the electrochemical tests include the requirements for comparatively expensive equipment (versus mass loss tests) and higher levels of technical expertise for data analysis. Fmthermore, the data reduction requires the use of conversion-"constants," factors applied to the results of the electrical measurements to convert the data to a corrosion rate. These constants ... 

Since it is often difficult to visualise the extent of attack in terms of depth from such mass-loss units as mdd, it is common practice to convert these mdd figures into others to indicate depth of penetration, i.e. inches per year (ipy), mils or mm y" . Such calculations suffer from the same defects as the mdd figures in that they take into account neither changes in corrosion rates with time nor non-uniform distribution of corrosion. However, since such conversions are often made it is desirable for the initial reporter of the test results to make the calculations accurately and to report corrosion rates in both mdd and mm y or similar units. 

It should be realized that the utOity of the tests is limited if the corrosion is not uniform. If there is extensive localized corrosion, the mass loss data will greatly underestimate the maximum corrosion rate, as it will distribute the corrosion over the entire surface of the coupon. Where localized corrosion may be a concern, the investigator may use a micrometer or other type of sensitive instrument to measure the coupon thickness before and after test. Multiple measurements should be made to ensure that the coupon surhice is well-characterized. The thickness loss in mils divided the test duration in years should be compared to the corrosion rates calculated by the above formula. The degree to which the two results agree indicates the degree to which the coupon did indeed undergo uniform corrosion. 

The decision to test is usually driven by unusual chemistry such as a new catalyst, reaction components, or reaction conditions. Immersion/mass loss method is primarily used in conjunction with microscopic examination. Process fluids from production or pilot runs are primarily used to best simulate potential corrosion. The test methods are custom designed based, in part, on cost and the ability to obtain sufficient quantities of test fluids, and handle the process conditions. Process conditions are tested outside the process control limits (e.g., temperatures, pH) to better accentuate the corrosion potential. These extremes have to be tempered by the stability of the products in the stream. For wall thicknesses greater than 0.250 in. (6.35 mm), a uniform loss of less than 10 mil/year (0.26 mm/year) is considered structurally acceptable. Signiflcandy lower levels of uniform loss are of concern for product or process contamination issues. Microscopic examination is used to determine potential localized corrosion concerns, such as pitting or stress cracking. Indications of pitting or stress corrosion in stamped areas of the coupon are of particular concern. U-bend tests are rarely used because of insufflcient test fluid quantities and availability. 

The type of corrosion, if there is corrosion, is detected visually. If the corrosion is uniform, the dissolution rate is determined by measuring the mass loss. This kind of test has evaluated the resistance of aluminium in many media, and the reported results are summarised in Parts E and F. 

Incorporation of nanosized SiC particles in the EL NiP matrix increased the hardness from 627 43 to 704 60HVqqj. However, incorporation of micronsized SiC particles increased the hardness to 1669HVqqj. The cumulative mass loss versus time (see Fig. 8.26) due to cavitation erosion corrosion in 3.5% NaCl is found to be very low for EL Ni-P-nano SiC composite coating compared with its coimterpart incorporating micron-sized SiC particles. The EL Ni-P-nano SiC coated specimen exhibits a smooth and uniform surface after the cavitation erosion corrosion test with no visible pits (see Fig. 8.27). The uniform distribution of the nanosized SiC particles in the EL NiP matrix (R 0.95 0.060 pm) inhibits the formation of pits. However, the higher surface roughness (R 1.72 0.051 pm) promotes bubble formation and causes detachment of the SiC particles, which results in the formation of small cavities on the surface of EL Ni-P-micro SiC coated steel (see Fig. 8.27). Hence, it is evident that incorporation of nanosized particles in EL NiP matrix could provide a better cavitation erosion corrosion resistance and inhibit the onset of erosion damage near surface defects. 

---