Page 231 - Materials Chemistry, Second Edition

218 3 Metals

post-deposited onto a metal surface in an effort to prevent corrosion. In this
section, we will discuss three strategies that may be used to protect a metal surface
from its environment: inert-layer passivation (either native (e.g.,Cr 2 O 3 on Cr) or
purposefully deposited), sacrificial metal coatings (e.g., galvanized steel), and organic-
based coatings. It should be noted that a pretreatment process is often required for
metal surfaces to allow the coatings to be strongly adsorbed to the surface. These steps
effectively remove organic components such as oils, as well as inorganic species
such as welding flux. The most common methods are either mechanical descaling
(e.g., abrasive blast techniques) or chemical pickling (i.e., acid treatment).
The primary corrosive agents are oxidizing agents (e.g., moist air, HNO 3 ,H 2 SO 4 ),
or halogenated species (e.g.,Cl 2 , HCl, HF, CFCs). These agents degrade the metal
by forming oxides, hydroxides, or halides that introduce embrittling grain boun-
daries on the surface. Other particularly detrimental gases for corrosion of metal
surfaces are CO and H 2 S. Both of these gases exhibit dissociative adsorption on
metal surfaces, resulting in carbide or sulfide formation and concomitant embrittle-
ment of the metal. In the presence of H 2 S(or H 2 O) at elevated temperatures, the
hydrogen atoms may also interact with surface metal sites and cause surface cracking.
Hydrogen-induced cracking is especially detrimental for iron surfaces.
For carbonaceous gases such as CO and CH 4 at relatively high temperatures

(ca. > 800 C), carburization of steel surfaces takes place in the form of brittle
interstitial carbides that may cause surface cracking. Cementite may also form on
the surface of steel; since its melting point is lower than the underlying metal, it may
cause melting of the steel surface that is subsequently eroded by the gas stream.
One simple method used to deter the onset of corrosion is phosphating. This
process is often used to chemically passivate a metal surface with a crystalline
coating of zinc phosphate. The phosphating bath is an aqueous solution of dilute
phosphoric acid, containing anionic and cationic elements that are capable of
reacting with the metallic surface to yield a crystalline film on this surface. Other
components of phosphating baths, known as accelerators, influence the kinetics of
the reaction process and permit the control of redox reactions at the interface.
The most common surface species present after phosphating are vivianite
[Fe 3 (PO 4 ) 2 ·4H 2 O], hopeite [Zn 3 (PO 4 ) 2 ·4H 2 O], and phosphophyllite [Zn 2 Fe
(PO 4 ) 2 ·4H 2 O]. For more complex substrates such as steel coated with Zn/Fe,
Zn/Ni, Zn/Al, and Zn/Cr alloys, tricationic phosphatings have been developed.
For these systems, the surface is coated with crystalline pseudophosphophyllite
[(Zn, M, Ni) 3 (PO 4 ) 2 ·4H 2 O], where M ¼ Fe, Al, Cr, etc. For aluminum containing
alloys, or hot-dipped galvanized steel (containing Al 2 O 3 on the surface), it is
necessary to use fluoride-based additives to cause surface crystallization.
Another useful passivation technique is anodizing or anodic oxidation. In this
method the metallic surface acts as an anode, being oxidized during an electrochem-
ical event. The most common metals/alloys are those containing aluminum, magne-
sium, and zinc. However, it is also possible to anodize other metals such as copper,
steel, and cadmium for protective and decorative applications. The anodizing
electrolytic solution consists of strong acids, generally combinations of chromic,