Page 30 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

8 INTRODUCTION AND FORMS OF CORROSION

(iii) Polarity Inversion. The properties of the electrolyte such as pH, potential, tem-
perature, fluid flow, concentration of different ions, dissolved gases, and con-
ductivity can change with time and influence the polarization, the properties of
the interface, and the galvanic potential of the components. The change in ion
activity of one metal can reverse the polarity of iron–tin couple for iron-plated
tin food cans. This shows the importance of media in galvanic corrosion where
the canned food boxes are made of iron with a coated inside layer of tin. The
tin may then react with food constituents forming a soluble complex. The con-
centration of tin is sufficient to the extent that it becomes anodic to that or iron
and begins to corrode. Also, iron is protected by zinc coating as a sacrificial
anode (galvanization) because of the formation of Zn(OH) and white rust,
2
∘
and at temperatures above 60 a hard compact ZnO layer is formed, which is
cathodic to iron, and this layer is able to reverse the polarity sign of the couple
Fe–Zn. This phenomenon is observed with Cu–Al or Ag–Cu couples (8).
(iv) Deposition Corrosion. Dissimilar metallic corrosion can occur following the
cementation of a more noble metal. In copper pipes carrying soft water con-
taining carbonic acid into galvanized tank, any dissolved copper ions can be
deposited according to the reaction

2
Cu + Zn → Cu + Zn 2+

which causes additional galvanic corrosion of zinc.
Severe corrosion may occur in active Al or Mg alloys in neutral solutions of
heavy metal salts (salts of Cu, Fe, or Ni). This type of corrosion occurs when the
heavy metal salts plate out to form active cathodes on the anodic magnesium
surface. This type of galvanic corrosion can lead to localized pitting corrosion.
(v) Hydrogen Cracking. Self-tapping of martensitic stainless steel screws attached
to aluminum roof in seacoast atmosphere failed by cracking. Hardened marten-
sitic stainless steel propellers coupled to steel hull of a ship failed by cracking
in service. Tantalum is embrittled by hydrogen on polarization or coupled to a
more active metal in an electrolyte (8).
(vi) High-Temperature Galvanic Corrosion. High-temperature galvanic corrosion
∘
is involved in the reaction of silver with gaseous iodine at 174 Cin1 atm
oxygen, which is accelerated by contact of silver with tantalum/platinum/
graphite (8).

1.2.1 Factors involved in Galvanic Corrosion
Electromotive force (emf) series and “practical nobility” of metals and metalloids
are given in Table 1.2. The emf series, also known as the Nernst scale of solution
potentials, are proportional to the free energy changes of the corresponding reversible
half-cell reactions with respect to the standard hydrogen electrode. The “thermo-
dynamic nobility” may differ from “practical nobility” because of the formation of
passive layer and electrochemical kinetics.