June 2011 Archives

There are wide variety of anodize layer types with varying characteristics for aluminum and aluminum alloys.
The factors that determine the surface layers depend on the base metal. The mechanism of anodized layer forming differs from the electro-plating where anodized layers are created by the base metal being electrolytically dissolved to form oxidized layers. The thicker the layer becomes, more base metal is dissolved. Additionally, the base metal properties dictate how easily the oxidized layers can be formed.
There are many types of aluminum alloys with various compositions depending on their purposes, and they can be divided into heat treated alloys and non heat treated alloys. From a machining perspective, there are plates, bars, rods that are rolled, and cast, and die-cast mateials.
Various levels of anodizing difficulties based on different alloys exist and a number of schemes have been developed for the difficult alloys. Generally practiced anodizing processes are shown on [Table 1] below.

[Table 1] Types of Anodize Processes

There are virtually infinite number of industrial anodizing processes combinations as the [Table 1] suggests, based on bath types, power supply waveforms, process speeds, and application purposes.
Currently, the most common electrolyte type is the sulfuric acid bath. The reason for the popularity is that the formed layer is colorless, and the power supply voltage and power consumption required for the electrolysis is low and economical for colored parts production. For specialized colors and wear resistance requirements, oxalic acid bath layers and oxalic acid added sulfuric acid bath are used.
Of the various electrolytes, the chromic acid bath is used almost only in the aerospace industry. As for the classification based on power supply waveforms, AC, AC superimposed, and pulse methods are used when the orthodox continuous DC does not yield satisfactory results.
Process speed classification in turn is a classification based on electrical current density. Large applied currents will form hard oxide layers by preventing the formed layer from dissolving into the electrolytes.
Various anodizing types discussed so far are in commercial use based on application requirements. Various aluminum materials will be discussed in the next volume.

Previously, electro and electroless plating were discussed. Anodizing process for light metals will be discussed from this volume.

Anodizing is a surface treatment process of creating an oxidized surface layer by an electrolysis process where the subject part is made and anode (+ pole) in an electrolyte solution. Processed aluminum and its alloy parts are called by well know name of "Alumite".
The anodizing process using various electrolytes such as sulfuric acid, oxalic acid, chromic acid, or organic acid, as well as different temperatures and power source electrical waveforms will produce surface layers of different properties.
The oxygen produced at the anode produces porous layer with good electrical isolation characteristics, corrosion resistance, and wear resistant properties. The layer porosity is utilized for coloring as well as various functions added, and used for products such as reflector plates, household pots and pans, construction materials, automotive components, machinery/optical/communications, and computer components.
In addition to aluminum, anodizing is used on magnesium, titanium, and tantalum, but are not for the same purpose as the processes on aluminum but for electrical components such as capacitors utilizing the nature of the surface oxidation layers.

The layers created by the anodizing process can be classified into two categories. One is the former Porous Layer Type represented by the "Alumite Process", and other is the Barrier Type used for capacitors. The layer properties for these two are completely different. In our discussion, the Porous Layer Type" will be explained for our purpose of Surface Treatment subject.

A lining process is where corrosion resistant materials are adhered to the inside of steel plating vessels and material transfer pipes in order to protect from corrosions. The thickness of the linings are normally several millimeters as opposed to 100 to several hundred micro-meters of painting, and the linings offer superior chemical stability and mechanical strengths compared to those of paints.

Surface treatment processes such as plating, etc. use various tubs such as plating tubs, de-greasing tubs, electrolytic tubs, neutralizing tubs, and washing tubs with strong acids, alkali, and highly oxidizing solutions. The tubs are made of steel to support the weights of these chemical solutions and subject materials. These tubs are lined with corrosion resistant linings to prevent the tub structures from corrosions and contamination of the contents.

[Table 1] shows currently used corrosion resistant lining material in frequent use. The linings are selected based on chemical resistance applicability.
In addition to the corrosion resistant characteristics, specific gravity and thermal conductivity are of significant importance. Low specific gravity equates to light weight for easier handling during the lining installation processes which also reduces overall costs. Lower thermal conductivity of the linings as compared to that of steel (48.3w/m-k) means the process liquid temperatures are maintained better.

[Table 1] Corrosion Resistance Comparisons of Lining Materials

Material Corrosion Resistant FRP Vinyl Chloride Neoprene Rubber Polypropylene Physical Properties Specific Gravity 1) 1.4-1.8 1.45 1.64 0.91 Tensile Strength 2) 85-180 50-60 20-35 25-380 Thermal Conductivity 3) 0.2558 0.1512 0.1163 0.0930 Corrosion Resistance Formic Acid 50％ or Less 40℃or Less Chloroacetic Acid 60℃ or Less Dilute Sulfuric Acid 60℃ or Less Concentrated Sulfuric Acid 90% or Less 60℃ or Less Norm. Temp. 50％ or Less Hydrochloric Acid 60℃ or Less Phosphate 60℃ or Less Chromic Acid 30％ or Less 60℃ or Less 3％70℃ or Less Hydrofluoric Acid 10％ or Less 25％ or Less Caustic Soda 50％ or Less 60℃ or Less 50％ or Less Ammonia Water 30％ or Less Sodium Hypochlorite

※	Note 1) g/cm3、Note 2) Mpa/mm2、Note 3) w/m・k、Good Corrosion Resistance、Poor Corrosion Resistance

Simply using the previously discussed corrosion resistant metals for structural materials in order to protect from corrosions will quickly become very expensive. Paints are used to protect from electrolytic and chemical corrosions by physically isolating the subject metals by surface coating with organic materials such as plastics.

Painting is applied on the subject metals by spraying, brushing, and roller coating. Subsequently, strong paint coats are formed by chemical polymerization of the ingredients. Some paints require 120 degrees C or higher temperature heating for the curing processes but natural curing paints are used for large structures such as steel beams, etc.

[Table 1] shows the general characteristics of corrosion resistant paints used in chemical plants and plating facilities.

[Table 1] General Characteristics of Paints

Resin Type Main Appli cation Water Resist ance Acid Resist ance Alkali Resist ance Sulfur Dioxide Resist ance Solvent Resist ance Heat Resist ance Wea ther Resist ance Alkyd Resin Steel Beams, Plates, General Vinyl Chloride Steel, Concrete Acrylic Building Exterior, Concrete Phenol Steel Beams, Chemical Plants Chlori nated Rubber Zinc Plated Surfaces, Bridges, Steel Towers Epoxy Ester Chemical Plants, Steel Epoxy + Amin Chemical Plants, Steel ○ Tar + Epoxy + Amin Water Proofing, Subme rged Objects × Poly iso cyanate Steel, Pro tection of Light Metals Thermo-plastic Acrylic Interior Alu minum, Steel Parts Nitro cellulose Wood, Steel × Silicon For heat resist ance ○ △ ×

※	Note : Excellent, Good, OK, Not Applicable