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MILANCO CHEMICALS
METAL CLEANING PART II
ALKALINE CLEANERS (AND ACID CLEANERS)
Milanco manufactures three types of metal cleaners falling into three main
categories:
1.
alkaline
2.
acid
3.
solvent
This course will deal mainly with alkaline cleaners, solvents and emulsions and a few acid cleaners. The iron phosphate cleaners which make up the bulk of the acid cleaners are covered in great length in the Phosphatizing Course. In this course, we will cover various metal cleaners, the differences between some cleaners, what to look for during an account survey, how Milanco’s' products are applied, how to choose the right cleaner and electrolytic cleaning.
Alkaline cleaner is a term given to a group of proprietary chemical blends which consists of alkaline salts, wetting agents and sequestering (chelating) agents. Alkaline salts cover a broad group of chemicals including caustic soda (NaOH), caustic potash (KOH), phosphate, silicates, carbonates and borates. This group of materials is said to be alkaline because they fall in the 8 to 14 pH scale. Just what is pH? This term is used often in the chemical industry. Therefore, it is important that you obtain as clear a picture as possible of its meaning, we will endeavor to avoid the chemist's definition which states, in complicated language, that pH is the measure of the INTENSITY of an acid or alkali. Just as amperage is the measure of electrical intensity or temperature the measure of heat intensity, pH measures the degree of intensity of acidity or alkalinity, This definition further states that pH is a numerical Iog function with each successive unit being ten times more intense. lf we wiII look to the accompanying chart, we might be better able to understand the underlying concept of pH chemistry.
The measuring scale of pH runs from 0 to 14 as indicated on the chart. The middle 7 in the series represents the neutral point. Pure water is neutral. Any number below this digit of 7 indicates acidity and any digit above this figure indicates the alkalinity of a solution. As the number get further from 7, the intensity of the acid or alkali becomes greater.
The following examples will give you an idea of how this log function works in practice.
EXAMPLE: If a tank of neutral water (pH 7) takes a pint of acid To bring the pH down to 6, it will take 10 pints to bring the pH from 7 to 5, and 100 pints to bring the pH from 7 to 4, etc. Naturally, the reverse on the pH scale is also true. If 1 pound of alkali increases the neutral pH of a solution to a pH value of 8, it would take 10 pounds to increase the pH from 7 to 9, and 100 pounds for a pH increase from 7 to 10. The factor of 10 continues all the way up and down the pH scale.
ANOTHER EXAMPLE: This factor of 10 is also applicable when neutralizing an acid or an alkali. An acid solution that would take 100 ounces of alkali to change the pH from 2 to 7 would require only 10 ounces to go from 3 to 7, 1 ounce for a change from 4 to 7, 1/10 ounce alkali to go from 5 to 7 and 1/100 for a change from 6 to 7. There are exceptions to this straight log function (10 factor) when certain acids are used because of the buffering action of the salts formed.
Two new words--salts and buffer--have entered our realm of chemistry, and of course, they have need for explanation. A salt is formed whenever an acid and an alkali are mixed,
HCL
+
NAOH
NACL
+
H20
Hydrochloric
Caustic
Sodium
chloride
Water
Acid
Soda
common
table salt
One item not indicated in the above reaction is the formation of heat. When an acid and an alkali are mixed a heat of reaction will be formed. Depending on the quantities and concentrations of these chemicals, care should always be exercised in their mixing as violent reactions and bubbling are possible, some reactions bordering on explosiveness.
Another word you will hear from time to time is buffer. We have discussed the formation of salts. Some of these salts resist pH changes in solution and are called buffers.
It's evident, from these examples, that the further the pH is from the neutral point of 7, the more difficult it is to change the pH. This explains why you cannot use pH as an indication of concentration.
Relative pH values of several alkaline materials:
0.1%
Solution-at 80°F
Caustic
soda
12
pH
Soda
Ash
11.6
pH
Phosphate
10.0
pH
Borax
8.5
pH
Each of the alkaline materials that are formulated into a cleaner is done so for the specific benefits that are rendered by that particular product. For example:
Silicates - form colloids (solids) which are insoluble but have excellent soil dispersing properties. In the presence of acids these materials become very insoluble and difficult to rinse. This could be highly unacceptable in the electroplating field for example.
Phosphates - noted as good hard water sequestrants Ich ors), excellent dispersion media.
Soda Ash - water conditioning--also provides reserve alkalinity.
Caustic Soda - Workhorse of the alkaline cleaners.
The second general group of chemicals which are incorporated in the make-up of a cleaner are wetting agents. Wetting agents fall into two categories, soaps and synthetic detergents. Soaps are long chain molecules, with one end of the wetter molecule being soluble in oil and the other end being soluble in water and holds the oil in solution by forming an emulsion of the oil droplets in the water. It is important to note that soaps can be made insoluble in hard water which causes them to lose detergency. Synthetic detergents are a large group of various complex chemicals. Synthetic detergents can act like soaps, but basically their job is to make water wetter. By this action, improvement of the penetration of water wettable soils is achieved. A lowering of surface tension at the metal-to-soil bond is facilitated by synthetic detergents. These qualities allow for more efficient and expedient soil removal.
The third and final group of chemicals generally formulated into a cleaner are the sequestering or chelating agents which have two functions:
1.
Softening water which makes for better rinsing and cleaning.
2.
To hold iron or other metals in solution, and keep it from redepositing on the part.
Alkaline cleaners depend on their detergency - lifting the soil from the surface by displacing it with surface active materials which are easily rinsed off - for cleaning.
We have discussed the three major components of a cleaner, especially an alkaline cleaner. But why do I use an alkaline cleaner? What are the benefits, if any?
Advantages of using an alkaline cleaner:
1. Very economical.
2. Acts on a variety of soils.
3. Broad spectrum of alkaline cleaners to choose from.
4. Leaves a desirable surface after cleaning.
5. Rinses with water.
6. Non-flammable.
7. Generally, little waste disposal problems.
Disadvantages of an alkaline cleaner:
1. Usually requires elevated temperatures.
2. Mechanical action necessary.
3. Undesirable effects on non-ferrous metals (caustic on aluminum).
4. Some soils cannot be dissolved by detergent.
5. Possible foam.
6. Slower than solvency.
There are three variable factors which determine cleaning -time, temperature and concentration. A change in any one of these three variables will necessitate a change in the other two. A decrease in temperature will necessitate an increase in time or concentration or both. Likewise, a shortening of the cleaning time will necessitate an increase in temperature or concentration of the cleaning solution. Acid cleaners are blends of mineral acids or their salts, solvents, wetters and inhibitors. The inhibitors control the action of the acids on certain metals. Acid cleaners are used mainly for rust and all types of scale removal.
Milanco manufactures a number of acid products. They basically fall into three types of acids, phosphoric, hydrochloric and sulfamic. These products are used by soaking, spraying or hand application for the following reasons:
1. Rust removal.
2. Heat treat scale.
3. Deoxidizing of metal surface.
4. Dissolving soil solids (boiler hard water scale, insoluble spray washer scale).
5. Brighten metal.
Here again, we have to consider the pros and cons in deciding whether or not to use this type of cleaner.
Advantages of an Acid Cleaner:
1. Can be economical (energy savings, fast reactivity).
2. Less metal loss.
3. Leaves smooth surface.
The third class of cleaners we will cover is Emulsion and Solvent cleaners. These cleaners can be either straight organic solvents or blends of organic solvents, emulsifiers and wetting agents. These cleaners mainly depend on their solvency (ability to dissolve the soil) for cleaning. Most of them leave a protective film against oxidation.
There are basically three types of solvent based cleaners:
1. Straight Petroleum (aliphatic, aromatic)
2. Non Flammable, Chlorinated
3. Emulsifiable solvents (petroleum solvents and surfactants and soaps)
The first two work on a straight solvency basis, the ability to dissolve a particular soil. the third class of cleaners works on the mechanism of emulsification which was discussed a little earlier.
In weighing one cleaner against another one should be familiar with the advantages and disadvantages of that particular class of cleaner.
Advantages of Solvent Cleaners:
1. High speed penetration - fast pre-cleaning.
2. Low temperature use - ambient to 150° F
3. Selectivity on certain soils - asphalt, tar, high melting waxes.
4. Loosens varnishes, paints.
5. Fast evaporation - removes excess cleaner from surface, leaves neutral surface.
6. Rust protection can be imparted by some products.
Disadvantages of Solvent Cleaners:
1. High cost
2. Residues may not be tolerated.
3. Flammable, some may be toxic.
4. Unsuitable for some soils.
5. May require special equipment.
6. Waste disposal could be a problem.
7. Buffing compounds - removal of a soluble component may leave inert abrasives which can become extremely difficult to remove in final cleaning. The use of slow solvents is recommended here.