The theory behind electrolysis as KATOLYSIS
Tests have proved that the protective layer is primarily formed by the negative aluminate ion.
By the anode H+-ions are formed (acid).
By the cathode OH- ions are formed (base).
Aluminium is dissolved in acid as well as in base.
Anode oxidation (recognised as electrolysis): Al Al++++ 3 e-
This positive aluminium ion will combine with the negative ions and salts in the water; mainly F- (high affinity), Ca(HCO3)2 with sulphate, which is not desirable, and with the formation of sludge; which happens mainly in the iso-electric point (see overleaf curve), and with OH-, which is formed by the cathode:
Al+++ + 4OH- Al(OH)4 – ( = AlO2- + H2O)
Furthermore, an important combination with silicate:
2 Al+++ + 2H2SiO4 + H2O Al2Si2O5(OH)4 + 6H+
The cathode reduction (Katolysis): Al + OH- + H2O AlO2- + 3/2 H2
As it can be read out of the formula, hydrogen will be formed. In praxis 0,028 ml H2/litre water, which equals a dissolved amount of Al of 0,05 mg. However, the solubility of H2 in water exceeds the produced amount, and accordingly it has no meaning.
The solubility of hydrogen is 21,4 ml at 0°C, and 8,5 ml at 80°C.
The aluminate ion is ready for reaction immediately when H+ occurs at the spot of corrosion in the pipe system. The reaction will be:
2H+ + 2AlO2- Al2O3 + H2O.
This results in an aluminium oxide precipitation on the metal surface as long as H+-ions and metal ions exist; i.e. only as long as active corrosion occurs.
Also the aluminate ion has a symbiotic relationship to silicate; which clearly favour the formation of the protective layer additionally:
2 Al(OH)4-+2H4SiO4 + 2H+ Al2Si2O5 (OH)4 + 7 H2O
The theory proves how much easier it is to form the protective layer by means of cathodically dissolved aluminium, which is active immediately; contrary to the anodically dissolved aluminium, which has to pass several chemical processes to become a negative ion.
On the curve (see overleaf), which shows the solubility of aluminium as a function of the pH-value, the difference between the two kinds of electrolysis is obvious. The anodically produced aluminium has to pass the iso-electric point at the bottom of the curve (due to the normal pH-value of potable water), where the tendency to precipitate is very high; contrary to the cathodically dissolved.
The curve shows that the anodically dissolved aluminium has the pass the bottom part of the curve – the iso-electric point – to become the same pH-value as normal for domestic water.
This results in the fact that most of it will precipitate as sludge.
With cathodic solution – KATOLYSIS – this will not happen with a pH-value exceeding 5,8: No sludge.
The curve shows a very low pH-value by the anode, and a corresponding high value by the cathode. These values, however, can only be measured directly by the electrode, and not in the water. Accordingly, no influence on the water takes place.