Page 31 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

GALVANIC CORROSION 9

TABLE 1.2 List of Some Systems Leading to SCC (9)
Alloy Environment
Aluminum alloys Aqueous chloride, cyanide, high-purity hot water
Carbon steels Aqueous amines, anhydrous ammonia, aqueous carbonate,
CO , aqueous hydroxides, nitrates
2
Copper alloys Aqueous amines, aqueous ammonia, hydrofluoric acid,
aqueous nitrates, aqueous nitrites, steam
Nickel alloys Aqueous chlorides, concentrated chlorides, boiling
chlorides, aqueous fluorides, concentrated hydroxides,
polythionic acids, high-purity hot water
Austenitic stainless steels Aqueous/concentrated chlorides, aqueous/concentrated
hydroxides, polythionic acids sulfides plus chlorides,
sulfurous acid
Duplex stainless steels Aqueous/concentrated chlorides, aqueous/concentrate
hydroxides, sulfides along with chlorides
Martensitic stainless steels Aqueous/concentrate hydroxides, aqueous nitrates, sulfides
plus chlorides
Titanium alloys Dry hot chlorides, hydrochloric acid, methanol plus halides,
fuming nitric acid, nitrogen dioxide
Zirconium alloys Aqueous bromine aqueous chloride, chlorinated solvents,
methanolic halides, concentrated nitric acid

Ignoring the kinetics and assuming that the passivating films are protective, the
practical nobility depends on (i) immunity and passivation domains, and (ii) the sta-
bility domain of water. Practical nobility is greater when the immunity and passivation
domains extend below and above the stability domain of water and the greater the
overlap of these domains with the part of the diagram between pH 4 and 10. Table 1.3
shows the classification of 43 elements according to thermodynamic stability and
practical nobility. This table of thermodynamic nobility and practical nobility must
be regarded as a guide as the electrochemical equilibrium diagrams are themselves
approximate in nature.

1.2.2 Galvanic Series and Corrosion
The practical change of the potential of the components of a galvanic couple as a
function of time is important. When the potential difference between two metals is
sufficient to form a sustained galvanic cell, the potential of every electrode can be
varied because of the active–passive behavior, the properties of the passive or cor-
rosion barriers, and the change in ion concentrations. The galvanic series is a list
of corrosion potentials, each of which is formed by the polarization of two or more
half-cell reactions to a common mixed potential, E measured with respect to a ref-
corr
erence electrode such as a calomel electrode. The galvanic series is a list of corrosion
potentials in seawater as shown in Figure 1.7. The material with the most negative