powder formulations

pretreatment
quality
powder selection
powder vs. liquid
racking
spray booth
spray gun
substrates
tape
types of powder

troubleshooting

The first step in the chemical pretreatment process is the removal of

oils, dirt, and other soils that will interfere with the development of a

good quality phosphate coating, good coating adhesion, or cause surface defects.

Chemical cleaning can be accomplished by subjecting a part to an aqueous spray or dip cleaner. The cleaner may be alkaline, acidic, neutral, solvent, or emulsion. The particular cleaner used will depend on the soils to be removed, the size and type of part, the type

of coating to be applied, and the substrate material.

The mechanisms for cleaning processes are generally recognized to

include solubilization, saponification, emulsification, sequestration and deflocculation. In each of these processes the action requires surface wetting of the metal by the cleaning solution.

Solubilization, the dissolving of soils into solution, can occur when the soils have very similar polarity and chemical affinity for the cleaning media.

Emulsification, the suspension of soils in solution, requires that the soils dispersible in the cleaning media.

Saponification, turning the soils into soap, applies specifically to those soils that contain carboxylic acid and ester functionality that can react with alkaline cleaning media.

Sequestration involves the deactivation of metallic ions in the soil to prevent them from interfering with the detergent action of the cleaner.

Deflocculation is a process that breaks up large particles of aggregate

soils into a finely divided material that is held in suspension in the

solution to prevent redeposition on the part surface. The last two

processes generally operate in conjunction with the first three processes mentioned.

While alkaline cleaners are the most common, there are also acid

cleaners and emulsion cleaners used for industrial applications. The

cleaner selected must have the ability to remove a wide variety of

soils, prevent redeposition, provide cleaning even when contaminated,

provide foam control, be easily rinsed and be cost effective.

Alkaline Cleaning

Alkaline cleaners are the most common method of soil removal for

metal preparation prior to the application of powder coating. Cleaners

based on sodium hydroxide (caustic) are very economical where

cleaning by saponification is desirable. Caustic cleaning media are

highly reactive on non-ferrous surfaces and they can cause over-etching on aluminum and zinc surfaces, possibly creating smut and adding zinc to the solution. Caustic cleaning residues are also difficult to rinse away, especially if the solution temperature is in the high range.

Alkali silicates are excellent for cleaners that are used on non-ferrous

surfaces. Silicates can provide good cleaning with minimal chemical

attack and they do a good job of soil emulsification. They are a little

more costly than alkalis and not easily rinsed.

Synthetic detergents and surfactants offer many variations in composition. In some cases they cost a little more than alkalis but they provide a longer bath life which offsets the higher raw material cost. With good performance, easier handling and disposal, and superior effectiveness over a wider array of metals, these products are a good solution for many systems.

Typically, a mild alkaline cleaner (pH of 9 to 10) will provide better

soil removal and longer bath life than a high caustic solution. Residues of alkali salts will kill the free acid, drop out metal salts and kill the phosphate bath. A mild alkaline cleaner, prior to the phosphate stage, will aid in the formation of a more uniform, dense phosphate coating, leading to better paint adhesion and corrosion protection.

If the cleaning is not adequate, it is usually better to increase the time

in the cleaner rather than the concentration. Two mild alkaline cleaner

stages are better than one high caustic stage. Mild alkaline cleaners

are good for multiple metals and they can be run at a wide variety of

temperatures. A higher pH cleaner may be necessary on occasion for

very difficult soils.

 An alkaline cleaner is typically comprised of:

. alkaline base

. surfactant/detergent package

. additives for

. defoaming

. minimizing attack on substrates

. coupling agents

. water conditioners

Alkaline Cleaner Component Functions

. Silicates - (sodium metalicate, sodium orthosilicate) High alkalinity,

good saponifier and dispersant, softens water by precipitation,

inhibits dissolution of zinc and aluminum. May leave a whitish

residue on parts in not properly rinsed.

. Phosphates - (trisodium phosphate, sodium tripolyphosphate,

tetrasodium pyrophosphate, disomium phosphate) Softens water

alkalinity, improves rinsing, saponifier.

. Carbonates - (sodium carbonate, sodium bicarbonate) Alkalinity,

good buffering.

. Hydroxides - (sodium hydroxide, potassium hydroxide) High alkalinity,

saponifier.

. Nitrites - (sodium nitrite) Minimizes oxidation of metal due to

cleaner drying.

. Chelants - (versene) Softens water, changes form of precipitation.

. Surfactants - Provide water/oil solubility. Enables cleaners to

work more efficiently by reducing surface tension at the metal

surface. Also, prevents part from drying between stages or the

spray washer.

. Defoamers - Control foam.

Acidic Cleaning

Acidic cleaning is based on attack of the metal surface by sulfuric,

hydrochloric, nitric, phosphoric, hydrofluoric, fluorboric, or chromic

acids and the various acid salts of these acids. They generally include

a surfactant package, metal ion sequestrants, alcoholic solvents, and

an inhibitor to prevent excess attack of the metal. They can be useful

for removal of light oxides, organic residues, persistent salts and other

soils that are readily dissolved in acid.

Because of the fact that acids are corrosive and therefore more difficult

to pump and handle, and because in some cases they are inferior

to alkaline cleaners on organic soils, they are much less commonly

used. For metals that are prone to hydrogen embrittlement, such as

alloy steels and high-carbon grades of steel, acid cleaning is not an

option. Acids can also react with some metals to form insoluble

byproducts that interfere with subsequent processes.

In a three-stage washer, the first stage combines the cleaning and iron phosphating. These solutions will typically be made up of phosphoric acid, a wetting agent, and an activator.

Acid solutions may also be used to remove scale or oxides in pickling

solutions. These solutions are relatively strong mineral acid solutions,

using sulfuric, hydrochloric, phosphoric and nitric acid. This type of

solution can be useful for removal of stubborn inorganic contamination.

One particularly good use is the removal of laser cut scale. Laser

cutting of steel will form an oxide layer that is resistant to alkaline

cleaning.

Pickling rates increase with higher acid concentration and higher temperature.

Excess concentration should be avoided because of the corrosive

nature of the solution and the risk of an overly aggressive attack

on the metal.

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