Surface attack

Underdeposit corrosion is not so much a single corrosion mechanism as it is a generic description of wastage beneath deposits. Attack may appear much the same beneath silt, precipitates, metal oxides, and debris. Differential oxygen concentration cell corrosion may appear much the same beneath all kinds of deposits. However, when deposits tend to directly interact with metal surfaces, attack is easier to recognize. 

Pits (if formed) tend to be shallower, rounder, and less severely undercut than surfaces attacked by strong acids (Fig. 7.16). Wasted... 

The 18% Ni maraging steels do not display passivity and normally undergo uniform surface attack in the common environments. Of more serious consequence, however, for all high strength steels, is the degree of susceptibility to stress corrosion cracking (s.c.c.). 

Passivated layer is imperfect because of surface attack from aggressive ions. 

The cohesion of the matrix and reinforcements can be damaged, even in the bulk, by moisture or chemical surface attack. The fluids can propagate to some depth by absorption and wicking consequently it is important that a skin protects the reinforcements from direct contact with the external environment. 

The weight, thickness, and hardness of each coupon was measured before and after designated periods of exposure to determine the type and rate of physical and mechanical change. The hardness was measured with a Type D Durometer Instrument in accordance with ASTM D2240, and visual observations were made to identify changes in color or form. In addition, scanning electron micrographs of specific coupons were used to further identify the type and rate of surface attack. 

Pitting corrosion is a general term that can be considered a visible sign of the results of concentration cell corrosion and of further induced-corrosion processes such as when chloride attack occurs. Although pits can also occur with acid corrosion, etc., under-deposit corrosion, of course, can also involve direct metal surface attack, from, say, biologically induced corrosion (but that is discussed separately). 

In the dark, the surface attack of CS2 is said to leave a rubbery insoluble fraction similar to polymeric sulfur. 

Cosmetic corrosion Surface attack at spots where paint is damaged related to (i) red rust, (ii) paint creep, (iii) chipping factors involved are (i) coating composition and thickness, (ii) surface treatment, (iii) type of paint damage, (iv) type of paint generally evaluated by (i) measuring length of underpaint creep, (ii) extent of rust formation or paint loss... 

Appropriate methods for the study of the materials surface have been treated by the keyword Surface Analytical Methods. Especially XPS is a powerful tool even for practical corrosion problems and other problems which may occur at surfaces or surface layers. In many cases one may give solutions to practical problems caused by contaminations, wrong treatment of the surfaces and unexpected surface attack due to changes in the environment. This method has been applied to many practical problems in the macroscopic and microscopic environments ranging from large metal constructions to the micro- and nanoworlds of electronics. 

Four techniques based on mass spectrometry are widely used for multi-elemental trace analysis of inorganic compounds in a wide range of sample types. These techniques are thermal ionization (TI), spark source (SS), glow discharge (GD) and inductively coupled plasma (ICP) mass spectrometry. In these techniques, atomization and ionization of the analysed sample are accomplished by volatilization from a heated surface, attack by electrical discharge, rare-gas ion sputtering and vaporization in a hot flame produced by inductive coupling. 

The anion CIO cannot be an electrophile one protonation makes it neutral HOCl but it becomes electrophilic enough to attack an alkene only after a second protonation. Attack of the chlorine end of ClOHj evidendy occurs from underneath the alkene, presumably because the two substituents (Me and AcO) are on the top surface. Attack by water must then be from the opposite (top) face but why does it occur at the less substituted carbon This is a conformational question we must get the trans diaxial product for the correct alignment of orbitals (Chapter 18). 

The dissolution of oxides and silicates in the presence of O-containing organic ligands that form bidentate complexes with surface metal ions is a function of the concentration of complexation sites on the surface. In some cases, the precursor site for detachment may involve protonation as well as surface complexation. Some data suggest that at high concentrations of ions that form strong surface complexes, the rate-controlling step may involve surface attack. 

Carbon and low-alloy steels and stainless steels are the most commonly affected materials. The morphologies created by MIC attack seem to be related to the organisms involved. Pinhole openings under nodules, accompanied by extensive tunneling, may be typical of Gallionella bacteria on stainless steels, while shallow surface attack beneath nodules or open pits may be typical of sulfate-reducing bacteria on stainless and carbon steels. 

Figure 12. (a) LM photo of cavitation bacterial attack of Pinus radiata wood cell walls. The cavities are generally oriented perpendicular to the long axis of the fibers and frequently develop from bordered pit (BP) areas. Bar 5.0 pm. (b) LM photo showing appearance of the angular cavities (AC) of cavitation attack produced within the S2 cell wall of tracheids. Bar 5.0 pm. (c) SEM shows where the S3 has collapsed during specimen preparation, giving the impression of surface attack. BP = bordered pit. Bar 2.0 pm. 

Under extreme low temperature operations of Diesel engines the radiator failures occurring seemed to involve very similar corrosion reactions. Apparently the tin layer on the tube surface converted to an amorphous, powder form, breaking the bond between tube, solder and header. Figure also shows evidence of surface attack on the solder layer, by conversion of tin to amorphous form. The corrosion inhibitor used in these systems was a borate-nitrite-mercaptobenzothiazole proprietary composition. 

LTHC means a selective surface attack in the form of corrosion pits that form under long-term operation cycles at lower temperatures (around 700°C-750 C). The research on engine operating conditions has proved that the higher the gas flux velocity, the slower the pit corrosion damage [18]. 

If there is a perfectly homogeneous surface attack and a removal of material or a scale growth is taking place solely and uniformly on the surface of the material, the process is denoted external corrosion. 

For homogenous surface attack by active corrosion the case simplifies to the recession as there is no scale growing and the remaining cross section is of virgin material. 

The detection of ions released from metallic implants is dependent on the technique used. Very minute amounts of ions can be detected by electrothermal atomic absorption spectroscopy (ET-AAS), which goes down to concentrations of the order of ng/g. Electrochemical methods enable the detection of extremely low corrosion current densities, below 1 xA/cm, corresponding to dissolution in the passive state. These rates of corrosion do not modify the aspect of the surface and are not normally considered as surface attack. 

Under normal humidity conditions, no surface attack should be expected during the fabrication and storage of... 

Most of these polymers are regarded as highly stable in water because they are insoluble, often semicrystalline, and absorb only very low levels of water. However, most of them will undergo surface attack in acids or alkalis and are generally unsatisfactory for long-term use in these media. The same is true of... 

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