What is the pH of water and its effect on corrosion processes?
The article describes the physical and mathematical meaning of the pH value of water, the conditions of corrosion processes with hydrogen depolarization, and the article also offers a hypothesis about the causes of self-ionization of water.
The water molecule consists of two ions, the hydrogen cation (H+) and the hydroxide anion (OH–), which under normal conditions form water molecules H+ +OH–= H2O, which are attracted to each other forming liquid water. At the same time, it is known that water has the property of self-ionization (autoprotolysis). This means that water contains not only water molecules directly, but also hydrogen cations and hydroxide anions in an ionized, i.e. charged, uncompensated state, which arose as a result of self-ionization.
In one liter of water the concentration of hydrogen and hydroxide ions is extremely low. It is known that water completely purified from ions has an extremely low electrical conductivity equal to 0.055 microCm/cm. No matter how many ions you remove from the water, it will not be possible to achieve an electrical conductivity value below the certain value. This suggests that the process of self-ionization occurs in water, and even after removing all cations and anions, some of the water molecules dissociate to form a hydrogen cation and a hydroxide anion. Perhaps this is due to the fact that the water is affected by a certain constant electromagnetic field that evenly fills the entire surrounding space. At least this could explain why self-ionization of dipole liquids occurs. Regarding the nature of a possible constant electromagnetic field that evenly fills the entire space, I wrote the book “Theory of massless interaction” [1].
Let’s calculate how many hydrogen and hydroxide ions are formed in water. The dissociation constant of water is Kw = 1.8 * 10-16 mol/l at 25 0C. It tells us what proportion of water molecules will be subject to ionization or, in this case, dissociation.
Кw=[H+]*[OH–]/[H2O] or Кw* [H2O] = [H+]*[OH–]
Let’s calculate how many moles of water are contained in one liter of water. The molar mass of water is 18 g/mol. Accordingly, 1000 (grams of water / one liter) / 18 (grams/mol) = 55.56 mol/l. We get the concentration [H2O] = 55.56 mol/l.
Thus,
1,8*10-16 * 55,56 = [H+]*[OH–]
or
10-14 = [H+]*[OH–]
If we assume that water is completely free of foreign ions, then it self-ionizes to form the same amount of hydrogen and hydroxide ions. The product of the molar concentrations of these ions is 10-14. Accordingly, the concentration of hydrogen ions is 10-7 and hydroxide ions 10-7. The mathematical meaning of the pH value of water follows from this condition. The pH value is an indicator of the degree of molar concentration of hydrogen ions in one liter of water taken without a minus, provided the concentration is expressed as 10, or the negative decimal logarithm of the concentration of hydrogen ions.
рН = – lg[Н+]
рН = – lg[10-7] = 7,0
If the water contains 10-7 mol/l of hydrogen cations and 10-7 mol/l of hydroxide anions, then the specific electrical conductivity of such water will be 0.055 microCm/cm.
The equivalent electrical conductivity of hydrogen with infinite dilution is equal to 349.8, and hydroxide is equal to 198.3. We get 10-7*349,8+10-7*198,3 = 0.000005481 mCm/cm or 0.05481 microCm/cm. Which confirms the correctness of the value of the dissociation constant of water.
The product of the concentrations of hydrogen and hydroxide is constant. That is, if the concentration of hydrogen ions in water increases, then the concentration of hydroxide ions decreases proportionally. For example, if you add acid to water and get a pH equal to one, it will mean that the concentration of hydrogen ions in a liter of water is 10-1 mol/l, and the concentration of hydroxide, respectively, will be 10-13 mol/l, because 10-1 * 10-13 = 10-14. We obtain a sufficiently large concentration of hydrogen cations 10-1 = 0.1 mol/l, and an extremely small concentration of hydroxide anions 0.001 nanomol/l. If you add alkali to the water, then everything will happen exactly the opposite. There will be a small concentration of hydrogen and a large concentration hydroxide.
The pH measurement process in water is organized in the following way. Ion-selective electrodes and reference or zero electrodes are used for this purpose. Hydrogen selective electrodes are used in this measurement. The ion-selective electrode is made of special glass and filled with 0.1 N hydrochloric acid. A potential arises between the hydrogen ions of hydrochloric acid and the hydrogen ions of the analyzed sample in the case of their different concentrations, which is fixed on the potentiometer through the reference electrode. The magnitude of this potential is the pH value.
It turns out that hydrogen cations are present in the water all the time, even at a pH close to 14. Hydrogen cations are the cause of metal corrosion with hydrogen depolarization. Consequently, even at values close to pH 14, corrosion can occur. But in practice, corrosion processes practically stop at pH values of more than 8.3. Although ionized hydrogen cations are present in the water. Probably, we can assume the following, if there is a predominance of hydroxide over hydrogen in water, then corrosion processes with hydrogen depolarization do not occur. It can be said that all hydrogen is bound by hydroxide and under normal conditions there is no driving force of the corrosion process or it is extremely small. Because the violation of the hydrogen–hydroxide ratio in water also requires a certain driving force, which probably balances the driving force of the corrosion process with hydrogen depolarization. But as soon as hydrogen appears in the water, not bound by hydroxide, i.e. the pH of the water drops below 7, then corrosion processes immediately begin to be observed. The lower the pH, the higher the corrosion rate, because there is more “free” hydrogen in the water.
Nevertheless, for almost any water, the pH value = 7 is not the limit above which corrosion processes with hydrogen cease to occur. The fact is that any water contains a carbonate buffer system. This system allows the water to remain for some time in the pH range acceptable for organic compounds from 4.5 to 8.5. This is a kind of mechanism for protecting organic life.
The carbonate buffer in water is formed by the following mechanism. Carbon dioxide enters the initially clean water. Carbon dioxide reacts with water molecules to form carbon dioxide (CO2+H2O = H2CO3=H++HCO–3), which in turn dissociates into hydrogen cation and bicarbonate anion HCO-3. Bicarbonate is an anion that is obtained from the binding of CO2 and OH (HCO3= CO2+OH). It turns out that when carbon dioxide comes into contact with water, a “free” hydrogen cation and a “bound” hydroxide anion are formed, which binds to the bicarbonate anion. Due to this, the pH of the water drops and hydrogen corrosion begins. This acidic water begins to react with carbonates (CaCO3 +H2CO3 = Ca(HCO3)2). This reaction is reversible. If there is more carbon dioxide than calcium carbonate, then all the calcium carbonate dissolves and passes into the dissolved form of calcium bicarbonate. At the same time, the carbon dioxide residue continues to ensure the flow of hydrogen corrosion. But interestingly, the pH value of such water can be higher than 7. But formally, there should be more hydroxide than hydrogen present in the water, and therefore hydrogen should be all bound and corrosion should stop. Indeed, corrosion almost stops from a pH value of more than 8.3. But at a pH value of, for example, 7.2, for water with a carbonate buffer system, quite active hydrogen corrosion will be observed.
Probably, it can be concluded here that the hydroxide bound to bicarbonate from the process of dissolving carbon dioxide in water is capture in water simultaneously, as it is part of the ionic product of water ([H+] * [OH–]). I.e., this hydroxide participates in the formation of the pH value of water, while being in the bicarbonate ion. Because if you start actively removing carbon dioxide from the water, the pH of the water will increase automatically. This means that the bond between the free hydrogen cation formed from the dissolution of carbon dioxide and hydroxide bound in the same process to bicarbonate also remains part of the ionic product of water. Only at pH = 8.37 all carbon dioxide is released or bound in water and free hydroxide (OH) begins to appear. Under these conditions, corrosion processes with hydrogen depolarization practically cease. In any case, they do not cause damage to equipment and pipelines.
It turns out to be an interesting situation. For example, for natural water with a carbonate buffer system, a pH value of 7.4 will mean that the water contains hydrogen cations = 10-7.4 mol/l, and hydroxide = 10-6.6 mol/l. That is, there is more hydroxide than hydrogen. But hydrogen corrosion in such water continues to occur and stops only at pH = 8.3. It turns out that free hydrogen passes into bound (it becomes the same or less hydroxide in the ionic product of water) when it reaches a concentration of 10-8.3 mol/l. In this case, the hydroxide concentration will be 10-14/10-8.3 = 10-5.7 mol/l (or pOH =5.7).
Water with a carbonate buffer system (any natural water) kind of stores its alkaline component OH and it continues to exhibit weak acidic properties at a pH of more than 7, but only up to 8.3. The carbonate buffer system avoids sharp acidification of water under unfavorable external conditions.
Based on these arguments, it is possible to make an assumption that natural water should be considered neutral, based not only on its pH, but primarily on the basis of the water saturation index (Langelier index). For example, the pH value of neutral water of deep rivers will be in the range from 7.4 to 7.9. If you reduce the pH of such water to 7.0, it will be quite corrosive. For thermal energy purposes, the pH value should definitely be above 8.3, which automatically requires water softening to prevent calcium carbonate precipitation.
List of referents:
- Теория безмассового взаимодействия/ Иван Тихонов. –[б.м.]: Издательские решения, 2022. – 150 с. link – https://ridero.ru/books/teoriya_bezmassovogo_vzaimodeistviya/
