Page 232 - Materials Chemistry, Second Edition

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3.3. Metal Surface Treatments for Corrosion Resistance

sulfuric, oxalic, or boric acids. The anodic layer obtained in sulfuric acid baths
consists of a relatively thin barrier layer, overlaid with a porous array. The density
and morphology of the pores may be varied through manipulation of the electrical
current or nature of the electrolytic solution. Although this presents a sufficient
limitation related to corrosion resistance, dyes may be added to the ca. 10–20 nm
diameter pores to yield a colored film. However, for protective applications, hydra-
tion sealing is often required which consists of steam treatment in the presence of
chromate or Ni/Co salts. In the case of aluminum, this post-treatment results in
boehmite, AlO(OH), that sufficiently seals the pores.
A variety of other nonmetallic coatings may be used to impart corrosion resis-
tance to the underlying metal surface. Common inorganic-based coatings include
vitreous enamels, ceramics, glass, cements, carbides, and nitrides. By contrast,
organic-based protectants are paint coatings, plastic coatings, adhesive tapes, and
sheet linings. Whereas the inorganic layers are often used to coat internal surfaces of
piping and reactors, organic films are most often used for external surface protec-
tion. Refractory coatings such as carbides (e.g., TiC, B 4 C, WC, and WCO), nitrides
(e.g., AlN and BN), oxides (e.g.,Al 2 O 3 ,BeO,Cr 2 O 3 , ThO 2 , and ZrO 2 ), silicides
(e.g., NbSi 2 , WSi 2 , and MoSi 2 ), and borides (e.g., ZrB 2 and TiB 2 ) impart both
corrosion/abrasive wear and temperature resistance to the underlying substrate.
More recently, an even greater corrosion resistance has been generated through
use of composite coatings, comprising the above refractory compounds in associa-
tion with a metal powder (e.g., Cr + ZrB 2 , Cr/SiC + HfO 2 , Al + SiC, Ti + TiB 2 ,
[22]
and PtRh + ZrB 2 ). For these coatings, the ceramic and metal powders are
suspended in an aqueous solution with the assistance of surfactants, and sprayed
onto the metal surface. This film is then allowed to dry at a temperature of ca. 70–
90 C, and annealed with an energetic laser source (e.g., Nd:YAG), resulting in

formation of an interwoven matrix of metal and ceramic species.
Without question, the easiest and most inexpensive method to protect metal
surfaces from corrosion is through simple painting. Paints comprise finely divided
solid inorganic or organic pigments (Figure 3.37) that are suspended in association
with binder molecules within a volatile solvent. In paints/varnishes, the nature of the
binder defines the type of paint system such as oil or water based, epoxy, etc.By
contrast, varnishes do not contain light-scattering pigments, resulting in a transpar-
ent coating. The volatile medium comprises solvents that are used to solubilize the
binder and dilutants whose purpose is to place the paint at a suitable viscosity for its
application. Solvents and dilutants normally comprise organic compounds such as
hydrocarbons, alcohols, ketones, ethers, and esters. Additives such as antifungal
agents, driers, etc. are often used to broaden the application of the coating.
Contrary to popular belief, a paint coating will not be completely impervious to
environmental agents surrounding the material, though this may be limited through use
of structurally complex pigments such as graphite, mica, aluminum oxide, etc.Due to
the incomplete blockage of corrosive agents, sacrificially active paint pigments are
often chosen specifically based on the corrosive agents that they will be in contact with.
For instance, impeding the corrosive ability of oxidizing agents can be achieved