Pitting prevention

Biostabiliser Incorporated into plastics Anti-fungi, algae and bacteria Prevent discoloration Stop cracking Prevent pitting Prevent brittleness Improve product life span Maintain aesthetic appeal Mediiun to long life... 

Fig. 5 Erosion pit inside a reducing pipe fitting, projection technique at 160 kV, profile plot with optical densities of the digitised film. The varying background caused by the geometrical set-up prevents a wall thickness calibration as in fig. 4...

In most cylindrical carbon—zinc cells, the zinc anode also serves as the container for the cell. The zinc can is made by drawing or extmsion. Mercury [7439-97-6J has traditionally been incorporated in the cell to improve the corrosion resistance of the anode, but the industry is in the process of removing this material because of environmental concerns. Corrosion prevention is especially important in cylindrical cells because of the tendency toward pitting of the zinc can which leads to perforation and electrolyte leakage. Other cell types, such as flat cells, do not suffer as much from this problem. 

Under cyclic or repeated stress conditions, rupture of protective oxide films that prevent corrosion takes place at a greater rate than that at which new protec tive films can be formed. Such a situation frequently resiilts in formation of anodic areas at the points of rupture these produce pits that serve as stress-concentration points for the origin or cracks that cause ultimate failure. 

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

Figure 2-11 shows weight loss rate-potential curves for aluminum in neutral saline solution under cathodic protection [36,39]. Aluminum and its alloys are passive in neutral waters but can suffer pitting corrosion in the presence of chloride ions which can be prevented by cathodic protection [10, 40-42]. In alkaline media which arise by cathodic polarization according to Eq. (2-19), the passivating oxide films are soluble ... 

Higher alloys. Some higher alloys pit to a greater degree than lower alloyed materials. Inconel 600 pits more than type. 304 in some salt solutions. Just adding alloy is not necessarily the answer or a sure preventative. 

Avoid openings into buildings, cellars, or pits within 2 m or the separation distances, whichever is greater. (Any gully or drain unavoidably within 2 m should have the opening securely covered or fitted with a water seal to prevent vapour ingress.)... 

Distribution box Serves to distribute the flow from the septic tank evenly to the absorption field or seepage pits. It is important that each trench or pit receive an equal amount of flow. This prevents overloading of one part of the system. Dissolved solids Chemical substances either organic or inorganic that are dissolved in a waste stream and constitute the residue when a sample is evaporated to dryness. 

Pickling and passivation of Monel and stainless steel pressure vessels should be specified to prevent deep pitting. 

For services where ambient or inlet temperamres may fall below 0°C, flashback protection is provided by a special seal drum or loop seal in the inlet line. This equipment is designed specifically for the particular liquid and vapor materials being flared. In these cases a 150 mm minimum water layer is included in the bottom of the pit to prevent oil seepage into the ground, and the hydrocarbon inlet distributor is mounted 150 mm above the water surface. Details of the inlet distributor are shown in Figure 5. 

The aggressive ions either break down the protective films or prevent their formation and, in effect, increase the corrosion rate. In presence of chloride and sulfate ions the corrosion attack is more localized and, as a result, causes deep pitting. Inhibitive ions, on the other hand, tend to limit the attack and decrease the corrosion rate by forming protective films. The film is similiar to adherent carbonate-containing rust, which polarizes the anodic areas. When aggressive and inhibitive ions are present together, the aggressive ions, if present in sufficient quantities, interfere with the deposition of the protective layers. 

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. 

Care should be taken in every stimulation circumstance to allow fluids to drain to the reserve pit. In the completion operation it is exceedingly difficult to accomplish this due to traffic. Because of this, the service company should provide leak free hoses, lines, and connections. Upon completion of job, the hoses should be drained to a common area for holding subsequent to introduction to the reserve pit. Every precaution should be taken to prevent accumulation of fluids on the pad proper, thereby posing a potential risk to groundwater and runoff of location. 

Upon elimination of the fluids, the liner to the pit is folded over the residual solids in a way to prevent fluid migration. The liner is then buried inplace. The operator may choose to remove the liner contents completely to preclude any future contamination. In the case of a producing well, the location is reclaimed up to the deadmen. The adjacent areas are contoured to provide for drainage away from the production facilities. In the case of a dryhole, the entire location is reclaimed to the initial condition. All of the reclaimed area should be ripped to enhance soil conductivity. The top soil is then spread over the reclaimed area followed by seeding. Local seed mixtures are broadcast to quicken reintroduction of native plants. 

Blowdown sumps should be constructed from brick and/or concrete and the blowdown lines should drain under gravity. Where the blowdown lines enter the sump they should turn to discharge downwards and the bottom of the sump should be protected below this area with a cast iron tray to prevent erosion. The drain or overflow from the pit should be at such a level to produce a weir effect, thus holding water for dilution. 

---