Historical Articles

METAL CLEANING WITH ALKALINE CLEANING SOLUTIONS

Summary of a paper published by E. M. Baker and Richard Schneidewind. The original paper was published in the Transactions of the American Electrochemical Society, 45, 327 (1924).

December, 1924

In cleaning metals, e. g. prior to electroplating, it may be necessary to remove (1) oxides, such as iron rust, (2) grit or similar foreign particles, and (3). oil and grease. The oxides are usually removed by pickling. Grit is held principally by the grease and is therefore detached when the latter is removed.

Grease may be removed by organic solvents such as gasoline and benzol, but these are usually expensive and inflammable and the fumes may be objectionable. In most cases therefore, alkaline solutions are employed. The principal compounds used in commercial cleaners are sodium hydroxide (caustic soda, NaOH), sodium carbonate (soda ash, Na2SiO3), rosin and soap.

It has been hard to estimate the relative efficiency of various cleaning solutions, owing to the difficulty of controlling the conditions and of determining just when the surface is clean. If however, some property of the solution could be measured which largely determines its value in cleaning, the effectiveness of various solutions could be readily compared.

It is now generally recognized that the principal process involved in the removal of grease by alkaline solutions, is "emulsification." (An emulsion is an intimate mixture of two liquids, in which the one liquid is suspended in the other in the form of very small drops. Thus, e. g. milk is an emulsion in which the minute drops of butter fat are in suspension. That the fat is not in true solution is shown by the fact that it rises to the surface as cream on standing.)

The tendency of any two liquids to form emulsions with each other, depends upon the "surface tension" existing at their point of contact. If this surface tension is high, as e. g. between water and oil, there will be very little tendency to form an emulsion, i. e. "water and oil will not mix." (The condition is illustrated by the "water-break" on greasy metal surfaces.) If however the surface tension is decreased, e. g. by adding alkali or soap to the water, there will be a greater tendency to form emulsions. Finally, if the surface tension between the two liquids disappears entirely, e. g. between oil and gasoline, the one is soluble ill the other and we obtain a "true solution," and not an emulsion.

In this paper a simple apparatus is described, by which the relative surface tensions between oil and certain solutions may be measured, and the relative cleaning efficiency of these solutions may thus be predicted. The operation consists simply in measuring the relative sizes of the drops formed when the solution flows from a small tube into oil under specified conditions.

Suppose that into each of three separate test tubes containing lubricating oil a small fine-bore or "capillary" tube is inserted, and that water is run into the oil in the first tube; into the second an alkaline solution (e. g. of sodium carbonate) and into the third an organic solvent such as gasoline. It will then be observed that the water as it issues from the capillary forms large drops, the alkaline solution smaller drops, and the gasoline will run in without forming separate drops. The decrease in the size of the drops therefore corresponds to the decrease in surface tension.

To measure the relative surface tensions, all that is necessary is to determine the relative size of the drops under uniform conditions. This was done in an apparatus similar to that just described, and the results are expressed in comparison with the size of the drops of distilled water under specified conditions. A commercial lubricating oil was used as typical of the usual oil to be removed.

The results obtained with solutions of various substances likely to be present in alkaline cleaners, showed that for chemically equivalent concentrations, the surface tension was reduced to the greatest extent by sodium hydroxide, tri-sodium phosphate, sodium carbonate, sodium silicate and soap, all of which are known to be valuable constituents of cleaning solutions. In most cases the results with solutions containing two compounds, e. g. sodium hydroxide and sodium phosphate, were about the average of the results with the separate solutions. However, a small addition of borax to trisodium phosphate decreased the effectiveness of the latter more than in proportion to the lower cleansing power of the borax.

It was found that the emulsification is greater in hot solutions such as are usually employed in cleaning. It was shown that this effect is due principally to the reduction in the "viscosity" of the oil at the higher temperature. Such an effect is illustrated by the fact that upon heating, any solid greases are converted into liquids, which can readily form emulsions.

Soap was found to have the greatest emulsifying action of any of the substances tested. One objection to the use of soap in cleaners, is that it produces too permanent an emulsion, and hence the solution must often be replaced or replenished. Trisodium phosphate, on the other hand, forms an emulsion which is less stable so that on standing quietly for a few hours, the oil rises to the surface, from which it can be skimmed off, without appreciable loss of the phosphate.

The investigation led to the following recommendations for the preparation and use of cleaners:

1. Sodium hydroxide and trisodium phosphate are the most effective constituents of cleaners. The latter is less likely to stain the metal and is more easily rinsed from the work than is sodium hydroxide.
2. A mixture of sodium hydroxide and disodium phosphate (Na2HPO4) in molecular proportions produces trisodium phosphate and is equally effective. If not uniformly mixed, some portions may contain free caustic, which may attack the hands.
3. Soaps are especially useful for removing grit, but are hard to rinse, and are relatively expensive. Very soluble soaps, e. g. linseed oil soaps, are better than the harder stearic acid soaps.
4. Glycerine is objectionable in cleaners, as it causes peeling, If fat is added to a strongly alkaline solution, "saponification" occurs, i.e. soap and glycerine are formed, This method should not however be used to produce the soap in cleaning solutions.
5. Sodium silicate (water glass) is an effective substitute for soap.
6. The cleaner should be kept at as high a temperature as possible,
7. Agitation aid cleansing. Air agitation however cools the solution.
8. Electrolytic cleaning is effective only if the current density it at least equal to 10 amp./sq. ft., and preferably higher. Cathode cleaning is desirable except when cleaning a nickel surface to be re-nickled, when anode cleaning is suggested.
9. The life of a cleaner may be increased by skimming the oil from the surface each morning, and allowing the heavy dirt to settle below a false bottom.

Abstracted by W. BLUM