Pitting characteristics

Tests for Pitting. A test W hich is in popular use to determine pitting characteristics is the 10 percent Feme Chloride test which can be conducted at room temperature. This test is usually used for stainless or alloy steels. 

E7.10. Compare pitting characteristics of the alloys listed in Table E7.7 [7], determine which alloy would pit, and why ... 

Gupta, R.K., Sukiman, N.L., Cavanaugh, M.K. et al. (2012) Metastable pitting characteristics of aluminium alloys measured using current transients during potentiostatic polarisation. Electrochimica Acta, 66, 245-254. 

The inferior performance of coated AA2024 is associated with the less efficient bonding of silanol groups to the metal surface as a result of the alloy composition and pitting characteristics, thus suggesting lower surface affinity for the studied silane-based treatments with respect to AA6082 and AA5083. 

The last example presented in this section deals with the pitting corrosion of Fe in CIO solutions. Perchlorate is less known as an aggressive ion but reveals some unique and remarkable characteristics with regard to pitting corrosion. For example, the critical pitting potential (1.46 V against a standard hydrogen electrode (SHE) for Fe/1 M NaClO ) can be measured with an accuracy of less than 4 mV [61] which is very unexpected if compared to... 

Depression method. At locations where natural or artificial depressions exist, it is often possible to use them effectively for land-filling operations. Canyons, ravines, dty borrow pits, and quarries have oeen used for this purpose. The techniques to place and compact solid wastes in depression landfills vary with the geometiy of the site, the characteristics of the cover material, the hydrology and geology of the site, and access of the site. 

Pit interiors are characteristically smooth and distinctly hemispherical, but become rougher on less-noble alloys. Pits tend to cluster together, overlapping to form irregularly dimpled surfaces. Frequently, a lightly etched aureole surrounds the pit clusters. These etched areas are often produced by shallow corrosion beneath deposit and slime masses that covered the sulfate reducers in service (Figs. 6.3 and 6.4A and B). 

Wastage may appear as general attack or as grooving, or it may be localized. Pitting almost never occurs. Each attack morphology is characteristic of concentration processes associated with corrosion. 

Impingement is corrosion caused by aerated water streams constricting metal surfaces. It is similar to erosion corrosion in which air bubbles take the place of particles. The pits formed by impingement attack have a characteristic tear drop shape. 

There are a variety of chemicals that are toxic and used in the drilling fluid makeup. Chromates and asbestos were once commonly used and are now off the market. A mud inventory should be kept for all drilling additives. Included in the inventory are the material safety data sheets (MSDS) that describe each material s pertinent characteristics. The chemicals found on the MSDS sheet should be compared with the priority pollutants and any material should be eliminated if a match is found. The chemicals should also be checked on arrival for breakage and returned to the vendor if defective packaging is found. All mud additives should be housed in a dry area and properly cared for to prevent waste. Chemicals should always be mixed in packaged proportions. Wasted chemicals, ejected to the reserve pit by untrained personnel, can present future liabilities to the operator. 

Figure 4-491. Nonideal buffer characteristics of a 0.10 M carbonate reserve pit fluid [228].

The fluids from the reserve pit may be hauled away from location for disposal, reclaimed insitu, or pumped into the wellbore given a dryhole. The operator of the wellsite is responsible for the transportation offsite of the drilling fluids. The fluids may be considered hazardous in nature due to the toxic characteristics of most drilling and completion fluids. 

Electrochemical noise A variety of related techniques are now available to monitor localized corrosion. No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analyzed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking is obtained. 

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