Corrosion zones

The effect of anions on the zones of corrosion and passivation can be exemplified by a comparison of the Pb-H20 and Pb-H20-S04 equilibrium diagrams (see Section 4.3, Figs. 4.13 and 4.14) and it can be seen that in the presence of SOl the corrosion zone corresponding with stability of... 

In contrast with the Pb-HjO system, it can be seen in Figure 4.13 that in the presence of SO " the corrosion zone in the region of low pH no longer exists, owing to the thermodynamic stability of PbSO. The Pb-HjO-COj system has been expressed in a similar pH/potential diagram in which account has been taken of insoluble carbonates and basic carbonates of lead. 

Typical steel structures exposed to marine environments are (i) marine piles (ii) offshore structures (iii) vessels and (iv) others. The corrosive zones encountered in marine environment are (i) atmospheric (ii) splash (iii) tidal (iv) submerged and (v) seabed-embedded zone. 

Figure 20.4 summarizes the different domains of potentials and chloride contents where pitting can initiate and propagate. The boundaries between these domains are indicative, since they depend on several factors such as pH and temperature. Domain A (corrosion zone) indicates the conditions that cause initiation and... 

Base Number by Color-Indicator Titration ASTM D 664, Test Method for Acid Number of Petroleum Products by Potentiometric Titration or ASTM D 4739, Test Method for Base Number Determination by Potentiometric Titration. Neutralization numbers greater than 0.5 mg KOH per g of crude oil are likely to result in corrosion of carbon steel. Some prefer to use a neutralization number based on the distillate fraction present in the corrosive zone. This results in a critical neutralization number of about 1.5 mg KOH per g [79,20],... 

The Zn-HjO (Fig. 1.17) diagramshows that extensive corrosion zones exist at both low and hi pH values (compare the very restricted corrosion zone in the Fe-H20 diagram at high pHs) similar zones in the region of low and high pH are obtained with other amphoteric metals such as aluminium, lead and tin. The diagram for Zn-H20 predicts with some accuracy the behaviour of the metal in practice, where it has been established that zinc corrodes rapidly outside the range pH 6-12 5 but is passive within... 

Normalization of pH, Out of a Corrosive Zone, After Application of a Basic Hair Relaxer Solution... 

Figure 45 Marangoni convection of slag film in the local corrosion zone of a Si02(s)-(Fe0-Si02) slag system showing typical slag line attack.

The surface tension of silica is less than that of FeO. Thus, dissolution of silica will result in a decrease in surface tension of the slag. The local corrosion zone is narrow. 

X in this equation is the penetration depth of the internal corrosion zone, which is a function of the square route of time. is the initial solute concentration and y is a stoichiometric factor. Dq is the diffusivity of the oxidant in metal A. NqDq is also called the oxygen permeability in metal A. The considerations covered so far show that, even under a dense oxide scale BX, internal oxide CX can be formed if the latter has a higher stability than the surface scale oxide BX. The reason for this is that the equilibrium oxygen partial pressure of the outer scale oxide BX is higher than the Pq needed for the formation of the more stable internal oxide CX, i.e., according to Eq. (2-30) sufficient oxygen is dissolved in A for CX formation. 

Fig. 10.23 The corrosion zones and methods of protection for an offshore platform.

The growth kinetics of the internal corrosion zone depend first of all on the partial pressure pXj of the corrosive species present at the metal surface and leading to internal corrosion. This dependence is described by Sievert s law [22] ... 

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