caps and plugs
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 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
. 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,
. Hydroxides - (sodium hydroxide, potassium hydroxide) High alkalinity,
. Nitrites - (sodium nitrite) Minimizes oxidation of metal due to
. 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
. Defoamers - Control foam.. Inhibitors - Minimize attack on metal.
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
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.
June 14, 2012