Pitted Surface

Pitting Phenomenon. Pitting is a deep narrow attack seen in a restricted oxygen environment that often causes rapid penetration of the metal substrate. One characteristic common to all forms of pitting is attack in a localized region that surrounded by a region of very little or no corrosion.1 2 

The restricted level of oxygen within the pit causes that area to become anodic while the external surfaces then become cathodic. Consequently, the metal at the base of the pit dissolves to form ions. These ions migrate to the mouth of the pit where they are hydrolyzed to form hydroxide deposits. As the hydroxyl ions are consumed at the mouth of the pit, the net acidity within the pit increases.1 

The following equations demonstrate the primary anodic reaction  

Initiation factors that are believed to contribute to pit initiation are (1) imperfections or defects in the metal oxide, passive, or protective layer between the metal substrate and its environment and (2) exposure of the metal substrate to an aggressive medium. 

Propagation the corrosion is driven by the potential difference between the anodic base of the pit and the cathodic area surrounding the pit. Furthermore, reaction products formed autocatalyze the propagation reaction and cause the environment within to become even more aggressive. 

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OccasionaHy, metals that have undergone ECM have a pitted surface, the remaining area being poHshed or matte. Pitting normally stems from gas... 

Figure 5.8 Heavily pitted surface of a mild steel cooling water return line. Note the deep localized metal loss surrounded by aureoles of rust. Pitting is common when oxygen concentration varies greatly over time.

Corrosion products and deposits. All sulfate reducers produce metal sulfides as corrosion products. Sulfide usually lines pits or is entrapped in material just above the pit surface. When freshly corroded surfaces are exposed to hydrochloric acid, the rotten-egg odor of hydrogen sulfide is easily detected. Rapid, spontaneous decomposition of metal sulfides occurs after sample removal, as water vapor in the air adsorbs onto metal surfaces and reacts with the metal sulfide. The metal sulfides are slowly converted to hydrogen sulfide gas, eventually removing all traces of sulfide (Fig. 6.11). Therefore, only freshly corroded surfaces contain appreciable sulfide. More sensitive spot tests using sodium azide are often successful at detecting metal sulfides at very low concentrations on surfaces. 

Compared with cathodic blisters, which can be recognized by their alkali content, anodic blisters can be easily overlooked. Intact blisters can be recognized by the slightly lower pH value of the hydrolyzed corrosion product. The pitted surface at a damaged blister cannot be distinguished from that formed at pores. 

This method is generally not capable of achieving a uniform standard of cleanliness on structural steel. It is not effective in removing intact mill scale or corrosion products from pitted surfaces. The durability of subsequent coats is therefore variable and unpredictable, and depends on the thoroughness of the operation and the exact nature of the contaminants left on the surface. The method should be confined to non-aggressive environments or where short-term durability is economically acceptable. 

Rollers are available up to 450 mm wide and are at their best on large unbroken surfaces, e.g. storage tanks, pipework etc. when application rates two to three times faster than brush application are achieved. Long-pile (up to 30 mm long) rollers are particularly useful on rough or pitted surfaces. 

Phase transition occurs at a state of thermodynamic equilibrium, inducing a change in the microstructure of atoms. However, corrosion is a typical nonequilibrium phenomenon accompanied by diffusion and reaction processes. We can also observe that this phenomenon is characterized by much larger scales of length than an atomic order (i.e., masses of a lot of atoms), which is obvious if we can see the morphological change in the pitted surface. 

Figure 6.18 A stereoscan of a fracture surface of a dental silicate cement. The debonded glass particle is to be identified by its pitted surface, the result of selective acid attack. Note the particulate nature of the matrix (Wilson et at., 1972).

In addition, from the analysis of the ac impedance spectra obtained from the previously pitted surfaces under open circuit conditions at room temperature in 0.5 M Na2S04 solution51 and from the in-situ pitted surfaces in Cl ion-containing thiosulfate solution at various applied potentials and solution temperatures,52 they verified that dr>ss is inversely proportional to the CPE exponent for capacitive charging process (Eq. 30)51 and the depression parameter for charge transfer process.52... 

Enclose the disposal pit with 1/2 in mesh hail screen attached to cover support posts, to keep children and animals from entering and debris from collecting on the pit surface. 

Horticulture Pit. Core samples of soil of about 100 g size to a depth of 10 cm and liquid samples of 500 mL were taken from eight sampling points spaced uniformly across the pit surface. Each sample was then solvent extracted individually and the pesticides and degradation products were separated and quantitated. 

The evidence for pesticide losses due to evaporation is as expected for the more volatile pesticides such as EPTC, butylate, trifluralin and alachlor. Losses of other pesticides due to evaporation from the pit surface are uncertain because similar results were obtained for analyses of samples taken upwind and above the pit. 

Plexiglas Gain Swollen, soft, pitted surface... 

Another problem with glass strippers is that they are typically oxide-selective. Glass is composed of a combination of oxides. Because some of these oxides may be more susceptible to specific chemical attack than others, the result is an uneven surface. Such a pitted surface gets dirty easier, and is more difficult to clean, and the greater surface area requires more time for removal of adsorbed gases (i.e., water) in vacuum systems. 

In addition to characterizing mass losses, textural features can be used to discern the mechanisms and chemical environment associated with silicate dissolution. It is widely accepted that pitted surfaces, such as that shown in Figure 5, indicate surface reaction whereas smooth rounded surfaces result from diffusion-controlled dissolution (Lasaga, 1998 see Chapter 5.03). 

Corrosion of horizontal refractory surfaces (e.g. the furnace bottom) again depends on the difference between the densities of the original and of the saturated melt. If dissolution of the refractory produces a solution of higher density, this remains stationary at the interface and corrosion proceeds by non-stationary diffusion. In the opposite case, the lighter solution will flow spontaneously upwards this process has to be compensated for by the downward flow of the higher-density melt, so that a system of cellular currents is established and non-uniform corrosion results, producing an unevenly pitted surface. 

Tabular cemented units are 0.2-3 m thick tabular bodies that commonly extend for hundreds of metres or more laterally. They can be divided into three types those with original sedimentary structures preserved (type 1) those in which sedimentary structures are not preserved, with tube, groove and pitted surface textures (type 2) and those in which some mix of both type 1 and type 2 characteristics are present (type 3). 

In the range of electrode potential more positive (more anodic) than the pitting potential, the pitting corrosion occurs in the presence of chloride ions and the metal dissolution at a pit, initially hemispherical, proceeds through the mode of electropolishing, in which concentrated chloride salts in an occluded pit solution will control the pit dissolution. It is likely that the polishing mode of metal dissolution proceeds in the presence of a metal salt layer on the pit surface in the salt-saturated pit solution. It was experimentally found with stainless steels in acid solution [54] that the pit dissolution current density, pit, is an exponential function of the electrode potential, E (Tafel equation) ... 

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