April 2011 Archives

#077 Corrosion Protection Measures - Anti-corrosives

In limited environments such as cooling systems for chemical plant systems, adequate amounts of anti-corrosive use to prevent metal corrosions is possible. The environments where the anti-corrosives can be effective are with water, acids, and humidity, and mainly for steel. There are some anti-corrosive intended for use with copper and copper alloys but in limited applications compared to steel.
The mechanism of anti-corrosives vary depending on the chemical and can be classified into the following three.

(1) Passivation Agents

Agents added to environments to promote passivation in order to prevent corrosions of metal surfaces is called Passivation Agents. Steel has a self-passivating nature in water but not in normal water containing air. Steel forms a passivated layer in water only with existence of stronger oxidant. Representative passivation agents of this type are chromate and sodium nitrite.
These agents are added to the recirculating cooling water at 50~100ppm, and evaporation latent heat at the cooling tower cools the system. However, chromate is no longer used due to recent environmental concerns on splashes and waste disposal problems.

(2) Corrosion Protection Layer Formed on Steel Surface

This is a corrosion protection method of forming layers with anti-corrosive agent itself or compounds of the agents and waterborne elements. When phosphate polymer and phosphonate are added to water, they dissolve well but they bond with calcium ions and separately added zinc ions and form insoluble coating layer on the steel surface. These are thick and porous hydroxide or oxide layers unlike the passivated layers, and inhibit corrosions by preventing diffusion actions of dissolved oxygen onto the steel surfaces. These agents are now used for open cooling systems in place of chromates.

(3) Layer Forming by Steel Surface Absorption

This is a molecular layer forming process by agents being absorbed by the metal surfaces. Typical example pf this is organic amines. The polarized section of the amines containing nitrogen adhere to the steel surface, and other non-polarized sections are oriented outward creating a water repelling structure. When the surface is covered with such absorbed layer, the overall electrical potential will become positive, and frequently used as an inhibitor (anti-dissolve agent) for pickling baths.

Absorbed Layer of Organic Amines

#076 Corrosion Protection Measures - Other

(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

#075 Corrosion Protection Measures - Metal Coating - 3

(1) Metal Thermal Spraying

This is called Metal Thermal Spraying where thermally liquefied metals using gas or electrical power are spray coated onto subject surfaces by clean air. This method using thermal spray guns can relatively easily apply metal coatings on wide variety of objects as large as gigantic bridges to as small as dentures.

Wire Melt Thermal Spray Gun

Not only the metals with low melting points such as Zn and AL, but high melting point metals such as stainless steels and Ni alloys can be used for the thermal spraying process. Further, by spraying materials of high hardness such as ceramics, hard surfaces with excellent wear characteristics can be obtained.

However, the coating surfaces obtained by the thermal spraying processes are porous and not appropriately corrosion resistant. The coated surfaces will need to be either re-liquefied or other proper processes will need to be applied to seal the open pores. Re-liquefaction process is where the coated surfaces are reheated with flames and such to fill all the open gaps. Another sealing method is painting. Paint sealing on particularly porous coatings improves the metal coating adhesion by penetrating the pores acting as anchor to the base.

For corrosion protection of steel structures, highly corrosion resistant zinc, aluminum, AN-AL alloys are used for spraying. These coating materials are generally pore-sealed and painted as bases for final paint coatings. The ZN thermal spray coating thicknesses of 80, 120, 160, 200μm, AL thicknesses of 80, 120, 160, 200, 300μm, and Zn-AL alloy thicknesses of 80, 100, 120μm are generally used for industrial purposes. The wire melt thermal spray type is the frequently used method but an arc melt spray is also used in some applications.

Wire Melt Thermal Spray System

Arc Thermal Spray System

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