Corrosion Modes

Another potential source of corrosion problems is in the manufacturing process used to produce the aircraft. Specifically, the assembly and finishing processes can determine whether a specific component will be subject to premature corrosion. Of particular importance is the proper surface pretreatment and application of protective coatings and sealants, which must offer long-term durability to provide adequate corrosion protection. 

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The presence of organisms (large or small) in proximity to corrosion by itself is not proof of biologically influenced corrosion, any more than a correlation of lunar phases with stock market fluctuations establishes a lunar-financial connection. It should be stressed vigorously that all evidence must be consistent with any single corrosion mode before a definitive diagnosis can be made (see Critical Factors above). Further, all alternative explanations must be carefully examined. 

SCC has been defined as failure by cracking under the combined action of corrosion and stress (Fig. 9.1). The stress and corrosion components interact S3mergistically to produce cracks, which initiate on the surface exposed to the corrodent and propagate in response to the stress state. They may run in any direction but are always perpendicular to the principal stress. Longitudinal or transverse crack orientations in tubes are common (Figs. 9.2 and 9.3). Occasionally, both longitudinal and transverse cracks are present on the same tube (Fig. 9.4). Less frequently, SCC is a secondary result of another primary corrosion mode. In such cases, the cracking, rather than the primary corrosion, may be the actual cause of failure (Fig. 9.5). 

A classic feature of erosion-corrosion is the directional character of the metal loss. The metal loss will be oriented along the direction of fluid flow or according to turbulence patterns. However, other corrosion modes may produce directionality in metal loss and could be confused... 

In conclusion, it is evident that most of the degradation issues in diffusion layers are related to the hydrophobic properties of the materials. Therefore, techniques that investigate the different degradation and corrosion modes that may affect the diffusion layers inside fuel cells are vital in order to design and choose optimal diffusion layers with the goal of achieving the best reliability and performance possible. 

After selection of the material, the corrosion scientist must play a role in designing the equipment so that the design is appropriate, and avoids corrosion modes due to inappropriate design. Improper design may result in galvanic corrosion, crevice corrosion,... 

Failure due to corrosion may also involve a combination of two corrosion modes. In analyzing a failure due to corrosion it is necessary to identify the corrosion mode tentatively by visual and metallographic methods, the corrosive agents, corrosion morphology, and to make a definitive identification of the corrosion product or scale. 

In order to suggest preventive measures it is necessary to identify the corrosive mode in the system. The following outline of laboratory studies should help to identify the major corrosive mode. 

Effect of seawater velocity on corrosion mode of a range of commercial alloys. Source Ref 51... 

Figure 3.16 Internal and external corrosion modes of oil tanks (7).

The corrosion modes of rock bolts are (i) uniform corrosion (ii) pitting or crevice corrosion (iii) galvanic corrosion. 

The main disadvantage of ER probes is that they only give valid data when the corrosion mode is uniform. These instruments are not suitable when corrosion is localized (pitting, cracking). This is its major limitation in concrete where corrosion initiates with pitting, particularly when chlorides are present. There is a trade-off between the sensitivity of the probe and its usable lifetime. The thinner the probe the more sensitive but the more quickly consumed. 

A fundamental distinction between corrosion modes is the division into active and passive corrosion. The active mode is characterized by the loss of material to the environment and results in the decrease of size and weight of the specimen. The loss may be in form of gaseous or dissolved species. 

It is interesting to note that different approaches [17,18] result in applicability limits, which specify partial pressure limits of <10 bar to be tolerable in applications with strong security requirements in the active corrosion mode unless reaction control retards the process. 

The related add-on challenge is to optimize materials for conjoint failure modes when conjoint, nonlinear and coupled corrosion processes occur, including mechanically induced modes (wear, fretting, fatigue, and creep). Another need is the ability to handle or anticipate changes in solution or processes with time and transitions in corrosion modes. 

Exploit the full capabilities of defect engineering to avoid one-, two-, and three-dimensional defects of critical sizes, and spacings that tri er certain corrosion modes and their spreading. This is especially necessary in classes of alloys where heterogeneity is unavoidable, as in the case of precipitation age-hardened alloys. 

As a final precaution, whatever the test atmosphere, single data points emd single analytical measurements can be potentially highly misleading. Corrosion rates and modes vary with time. Data as a function of extended time are vital to define true performance in engineering applications. Because of the multiplicity of corrosion modes that can occur simultaneously in many environments, it is important to use as memy analytical techniques as practical to fiiUy understand all factors affecting performance. 

Resistance to SCC is of prime importance in structural, load bearing applications. Considerable testing is performed to ensure adequate resistance because failure by this corrosion mode could be catastrophic, and because of the uncertainty of the time to initiate a failure. 

Environment Corrosion Mode of Concern Potentially Susceptible Ti Alloys Comments... 

The framework summarized in Table 7.3, which was initially developed to predict the occurrence of stress corrosion cracking (SCC), was extended to other corrosion modes/forms. Additionally, an empirical correlation was established between the factors listed in Table 7.3 and the forms of corrosion described earlier in Chap. 6. Recognized corrosion experts were invited to complete an opinion poll listing the main subfactors and the common forms of corrosion as illustrated in the example shown in Fig. 7.9. Background information on the factors and forms of corrosion was attached to the survey. A total of sixteen opinion polls were completed in this survey and subsequently analyzed. 

The next step in the materials selection process is to discount the forms of corrosion that are unlikely to occur. For example, if the application being designed will not be exposed to a flowing fluid, then erosion-corrosion would not be considered. To determine the more likely forms that will occur, the analyst should examine the factors responsible for initiating each corrosion mode and then determine which ones are present in the application. Chapters 6 and 7 provide descriptions that could be quite useful for this phase of the selection process. 

Once the candidate materials and the environmental conditions have been determined, the analyst must investigate the potential for one or more of the many forms of corrosion to become active. At this point in the process the designer should have at least one candidate material to consider and a listing of potential corrosion modes in the anticipated environmental conditions. Since the scope of the analysis has been defined, one can assess whether any of the potential forms of corrosion may become active when the candidate material is subjected to the operational environment. 

Dissolution of the solid material in the liquid metal. The interaction product in that case is liquid under the form of the dissolved species in the liquid metal. This corrosion mode always exists even if the solubilities of the elements from the solid material are extremely low when there is a direct contact between the solid material and the liquid metal which is not prevented by the presence of intermetallic compounds or compounds, like oxides, formed between the dissolved species of the liquid metal and the solid material. 

Figure 17.14 Observed corrosion modes for Fe-Cr-Ni steels in stagnant LBE, depending on the inverse of temperature and on the dissolved oxygen concentration in LBE [40].

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