General attack

Stainless steels tend to pit in acid solutions. Pits form local areas of metal loss associated with breakdown of a protective oxide layer. Breakdown is stimulated by low pH as well as by the decrease of dissolved oxygen in occluded regions. Small, active pit sites form and remain stable because of the large ratio of cathodic surface area (unattacked metal surface) to the pit area. Active corrosion in the pit cathodically protects immediately adjacent areas. If conditions become very severe, pitting will give way to general attack as more and more of the surface becomes actively involved in corrosion. 

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. 

Ail homogeneous metals without differences in potential between any points on their surfaces are subject to this type of general attack under some conditions. Uniform corrosion is usually characterized by a chemical or electrochemical attack over the entire exposed surface, Figure 4-423. Metal corrodes in an even... 

Fig. I.S4 Pits formed during anodic polarisation in an acetic acid-acetate buffer of pH 4-6 containing thiourra or NaCI. (a) General attack and formation of crystallographic pits on nickel in the buffer -t- 10 m thiourea (x 200), (ft) crystallograhic pits formed in the buffer -H0 M...

Finally, it should be added that the extensive field tests made in the United States indicate that buried steel rusts less and less rapidly as time goes on, both as regards general attack and pitting. This can be illustrated by the typical results shown in Fig. 3.5. Field tests made in British soils by BISRA have not, however, exhibited the same tendency in these rusting has been roughly proportional to the duration of burial. 

Few general statements can be made regarding the effect on corrosion resistance of alloying elements or impurities. A useful summary of the information has been prepared by Whitaker. Copper is usually harmful causing increased susceptibility to intercrystalline or general attack, so that alloys... 

Lead is not generally attacked rapidly by salt solutions (especially the salts of the acids to which it is resistant). The action of nitrates and salts such as potassium and sodium chloride may be rapid. In potassium chloride the corrosion rate increases with concentration to a maximum in 0.05m solution, decreases with a higher concentration, and increases again in 2m solution. Only loosely adherent deposits are formed. In potassium bromide adherent deposits are formed, and the corrosion rate increases with concentration. The attack in potassium iodide is slow in concentrations up to 0.1m but in concentrated solutions rapid attack occurs, probably owing to the formation of soluble KPblj. In dilute potassium nitrate solutions (0.001 m and below) the corrosion product is yellow and is probably a mixture of Pb(OH)2 and PbO, which is poorly adherent. At higher concentrations the corrosion product is more adherent and corrosion is somewhat reduced Details of the corrosion behaviour of lead in various solutions of salts are given in Figure 4.16. 

Cr, Br , I, which cause pitting attack, and anions which form soluble complexes with aluminium , e.g. citrate and tartrate, which cause general attack. Competitive effects , similar to those observed on iron, are observed in the action of mixtures of inhibitive anions and chloride ions on aluminium. The inhibition of aluminium corrosion by anions exhibits both an upper and a lower pH limit. The pH range for inhibition depends upon the nature of the anion . 

Monosubstituted benzene derivatives are generally attacked in the para-position by trichlorocyclopropenium cation for toluene it was recently found39) that orthoattack also may occur as indicated by formation of 35 besides the normal product 34 ... 

Polar reagents AY (Section V,B-D) generally attack both triply bonded atoms of iminoboranes to yield aminoboranes [Eq. (27a)]. In special cases, the cationic fragment A of AY is added to the nitrogen atom and Y remains a separate anion [Eq. (27b)] such a reaction path seems to be governed by steric factors, but seems also to be restricted to aminoiminoboranes (70). 

Previous discussions (20, 39) on the propagation rate, kp, point out the effect caused by the resonance energy of the radical formed. Our results support this view and enable us to complete the arrangement by families according to the groups adjacent to the attacked function—alkyl, benzyl, alkoxy, allyl, hydroxyl. The steric effect does not reveal itself in any important way—e.g., a-methylbenzylic ether has a kp which is close to that of benzylic ether, and the tertiary carbons in the former product are generally attacked at rates comparable with that of a less-encumbered carbon. 

Cultivars of Cucumis melo L. can be attacked by Didymella bryoniae Auersew, which can cause considerable losses, because fruits attacked by this organism do not have commercial value anymore. Mosquitoes, mainly Bemisia tabaci, also attack melon cultivars. Reductions in weight, size, and sugar content are evident consequences of mosquitoes attack. Diaphania nitidalis attacks flowers and fruits, whereas Diaphania hyalinata generally attacks the leaves of the melon tree. 

The last type of partial seizure is the secondarily generalized attack, in which a partial seizure immediately precedes a generalized tonic-clonic (grand mal) seizure. This seizure type is described in the text that follows. 

Rubber latex cement withstands dilute acids and dilute bases, and adheres well to ceramic materials such as stoneware. This cement remains somewhat pliable, thus resisting mechanical shock and temperature change. Organic liquids in general attack this cement. 

The rate of attack for immersed conditions is fairly uniform in unpolluted sea water and averages about 5 mils per year. General attack, when it occurs, has been observed to be a linear function of time—that is, it continues at a fairly uniform rate, despite an accumulation of corrosion products or marine growth. 

Allylic electrophiles react readily with Pd(0) complexes to yield rf-allyl Pd(II) complexes, which retain electrophilic character and react with nucleophiles to yield the product of allylic substitution and Pd(0). Thus, catalytic amounts of Pd(0) provide an additional mechanism (in addition to SnI, Sn2, and Sn2 ) by which an allylic substitution can proceed. Because the metal generally attacks the allylic electrophile... 

Despite these potential difficulties, efforts to attack this problem have been undertaken and some progress has been made. The nonlinear equations are generally attacked by methods (e.g. Newton-Raphson) which require periodic solution of linear equations. 

In the reacting moiety of many cyclic or bicyclic molecules a stereostructure is present in which one can identify a convex and a concave side. Because reactions usually take place in such a way that the attacking reagent is exposed to the least possible steric hindrance, convex/concave substrates are generally attacked on their convex side. 

Electrophilic reagents generally attack carbon atoms 3 or 4 which are particularly reactive in 1,6-dioxa-6aA 4- thiapentalenes. 

Nucleophilic reagents generally attack at positions 2 or 5. Their action may be limited to the replacement of a substituent (generally SMe) or may lead to a modification of the triatomic sequence (1,6,6a) or may even cause a rearrangement to a y-pyrone. 

Systems cured with DETA and TETA generally have excellent resistance to aqueous sodium hydroxide even at 50% concentration and 80°C. Their resistance to 25 percent sulfuric acid, 23 percent hydrochloric acid, and 25 percent chromic acid is also very good up to 80°C. Strong organic acids such as acetic acid generally attack DETA and TETA cured epoxy resins as well as strong (40%) nitric acid. Resistance to intermittent solvent exposure is excellent however, long-term immersion in any aliphatic solvent such as kerosene is not recommended. 

Flattened three-, four-, or five-membered rings, especially ones with two or more trigonal carbons in the ring, are generally attacked from the less hindered face... 

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