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Surface Finishing Tutorial

#076 Corrosion Protection Measures - Other

Category : Corrosion - Corrosion Protection
April15, 2011

(1) Metallic Cementation Method

A method of creating a layer of alloys on metal surfaces by utilizing metal diffusion phenomena is called "Metallic Cementation". AL diffusion onto steel is called Calorizing, Zn diffusion is Sherardizing, and Cr diffusion is called Chromizing.
In order for these diffusion phenomena to occur, high heat furnaces are used and objects that are too large to fit in the furnaces cannot processed. Commonly seen items treated in this manner are shaped metals such as angle/channel iron and wire conduits. These objects are placed in sealed steel furnaces along with the metals to be diffused (for Zn diffusion, powdered zinc and zinc oxide compound) and fired at high temperature (300~400 deg. C for Zn diffusion). The diffusion process takes some long time and the tubing will be in annealed state with lowered hardness.
The electrical wire conduits, as seen on train station ceiling installations, are run in all three dimensional directions. The conduits are required to be flexible and corrosion resistant to withstand a long time usage.

(2) Chemical Conversion Coating

Coating by chemical reactions induced on metal surfaces is called :Chemical Conversion" method.
Phosphate coatings are frequently applied on AL, Zn, and Mg, but most commonly applied on steel. The coating is used not alone but as adhesion enhancing base coats for painting. This coatings is formed by boiling the subject steel in acidic phosphate solution to deposit Fe and Mn or Zn phosphate layers.
To create oxidation layers on aluminum surfaces, there is a method of boiling in alkali, but more commonly anodizing is used where AL as anode immersed in acidic bath is electrolytically processed. The oxidized layer by anodizing is porous and post-process sealing for improved corrosion resistance is always required.
Aluminum products, for light weight and good heat conductive properties, are widely used for aerospace, railway vehicles, automobile, and construction applications.
Anodized aluminum products are frequently used on curtain walls and window frames of buildings and houses where they are likely come in contact with mortar and alkaline liquids. For this reason, anodized parts are coated by electro-deposited water based painting to make them acidic rain and alkaline liquids. This process is called Compounded Anodizing Coating.

Anodizing Process of Shape Aluminum