Materials samples, corrosion behavior

Coupons must be identifiable before and after testing. Most simply, the coupons can be identified by a mark made directly onto the sample. However, with some notch-sensitive materials such as stainless steels that may have a tendency to favor localized corrosion over uniform corrosion, such a mark can modify the sample corrosion behavior. For the most sensitive materials, even a tag attached to the sample can affect the corrosion. In these cases, the materials should be identified by location within a test environment. Of course, the larger the test sample, the less of an influence some small mark will have on the overall results. 

Figure 13. Corrosion behavior of reduction reactor materials samples in anhydrous environment (O), Hastelloy C276 Cartech CB3 (V), Incoloy 825 (A), Inconel 625 (O), SS 310 and (0), SS 18-18-2. Furnace temperature, 482°C (900°F) anhydrous S03 12 cc/min argon 128 cc/min. Erratic erosion rate behavior of SS 310 and Cartech 20CB3 is caused by the spalling of the corrosion product. Negative values indicate weight gain per unit area.

Figure 14. Corrosion behavior of reduction reactor materials samples ( 7), Inconel 625 (O), silicon (Q), silicon nitride (%), alonized Inconel 62 (M silicon carbide and ( f), Inconel 657. Furnace temperature, 482°C S03, 25 see/min steam, 58 see/min argon, 78 see/min.

Potentiostatic polarization is widely used to determine the steady-state corrosion behavior of metals and alloys as a function of potential in environments of interest. This technique involves holding a specimen s surface at a series of constant potentials versus a reference electrode, and then measuring the current necessary to maintain each of the applied potentials. From this dependence of the current on the applied potential of the sample, a number of parameters important for understanding the corrosion behavior of the material in the environment (such as icon, b3, and bc) can be determined as pointed out in Chapter 2. 

A further complication is that these variations may not affect the deterioration of materials in a simple manner. In many cases, metal corrosion is dependent upon surface 61ms that form upon exposure to seawater. Variations of irutial exposure conditions can affect the protective nature of these 61ms, which can affect the further corrosion behavior of the material. Thus, a sample 6rst exposed under conditions conducive to the formation of a protective 61m may perform very differently than the same material exposed for an identical period, but was ffrst exposed to conditions that resulted in the formation of a 61m that was less protective. In other cases, short-term variations in environment may be sufficient to initiate attack that would not occur during typical conditions and that, once initiated, can continue over extended exposures. 

Since the corrosion behavior of materials in organic liquids can be influenced by low levels of water, minimizing the introduction of water from the ambient atmosphere during the introduction of samples or probes should be carefully considered. For long-term testing at sites where there is a constant turnover of solution, these effects may be small for a one-time insertion of coupons. However, if, for example, a retractable electrochemical probe is used, the solution with which it comes into contact should be flushed in order to allow it to sample the most relevant environment. 

In order to better understand the below melt line corrosion behavior, a crucible test was performed next. The crucibles had approximately 3 inside diameter and approximately 2.25" depth of cavity. Therefore, this sample represents a large area and is more representative of coarse-grained refractory products. Figure 6 shows the bonded and fusion-cast AZS samples, cut in half, following a corrosion test at 1427°C for 96h. Although the bonded AZS sample showed more material loss at the metal line compared to fusion-cast AZS sample, the bottom sur ce of both crucibles showed no material loss and similar visual appearance. 

No significant change was observed in the corrosion behavior of material samples in melt due to the presence of 0.5 mol% P11F3 addition in 15 LiF-58 NaF-27 BeF2... 

The results indicate an active corrosion behavior (i.e., silica being leached out of the samples). However, the corrosion rates of the SiOC ceramic materials were found to be remarkably lower than those of silicon carbide and were comparable to values reported for silicon nitride. Thus, a corrosion rate of 0.13 mg cm h was determined for the SiOC sample upon corrosion at 250 °C, being ca. 5 orders of... 

In conclusion, the results presented in this chapter demonstrate the extreme versatility of AW devices for the characterization of materials. The inherent sensitivity of AW properties to the mechanical and electrical properties of thin films can be used to advantage to directly monitor a wide variety of film properties. Since the properties and behavior of thin-film materials can be very different from those of similar bulk materials, this ability to directly measure thin film properties can be a significant advantage in materials research and development. The ability to use thin films instead of bulk samples has the added advantage that the time required to perform an evaluation of dynamic processes such as diffusion and corrosion can be greatly decreased. The number of applications of AW devices to thin-film characterization continues to increase, and is limited only by the ingenuity of AW device researchers and developers. 

The term surface analysis is used to mean the characterization of the chemical and physical properties of the surface layer of solid materials. The surface layer of a solid usually differs in chemical composition and in physical properties from the bulk solid material. A common example is the thin layer of oxide that forms on the surface of many metals such as aluminum upon contact of the surface with oxygen in air. The thickness of the surface layer that can be studied depends on the instrumental method. This layer may vary from one atom deep, an atomic monolayer, to 100-1000 nm deep, depending on the technique used. Surface analysis has become increasingly important because our understanding of the behavior of materials has grown. The nature of the surface layer often controls important material behavior, such as resistance to corrosion. The various surface analysis methods reveal the elements present, the distribution of the elements, and sometimes the chemical forms of the elements in a surface layer. Chemical speciation is possible when multiple siuface techniques are used to study a sample. 

A critical research gap in corrosion science is the absence of the corrosion equivalent for the stress intensity factor (K) that has been the mainstay of structural mechanics for the past several decades. The stress intensity factor was developed to predict the behavior of pre-existing flaws in structural materials and the eventual life of a component under conditions in which the flaw develops into stable cracks. The power of K is in the concept of similitude well-defined cracks and crack tips that are different in size or shape but possess the same K (as determined by geometry, loading, and the theories of linear-elastic fracture mechanics) will experience the same mechanical driving force for crack growth. Thus, similitude allows small, well-defined samples to be tested in the laboratory to determine the conditions of crack growth and fracture and the results to be quantitatively extended to more complicated real-world structures containing cracks. Virtually... 

As with many other materials, nickel and nickel alloys can be evaluated for uniform corrosion resistance according to guidelines in ASTM G 31. Due to the active/passive nature of many nickel alloys, the length of the test exposure can affect test results and reproducibility. In a short test lasting only a few hours, duphcate specimens can provide very different corrosion rates in some environments, like sulfuric acid. It is not unusual for one sample to show active behavior cuid another passive behavior over short time periods. Thus, extended test periods of about 100-200 h often provide more accurate results than shorter tests. The following... 

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