In chemical plants, the environmental conditions under which hydrogen embrittlement occurs can be roughly divided into: non oxidizing acids with a pH value below 3; Strong alkaline solution with a pH value higher than 12; Organic acids with pH=3; Salt solution containing F - and I -, neutral solution containing H2S. Under the above conditions, some cases are due to uniform corrosion, while others are related to local corrosion, but hydrogen is generated by electrochemical reactions. Of course, there is also another type of environment that already contains hydrogen.
For titanium condensers or condensers cooled by seawater, when titanium metal is mixed with aluminum brass tubes or titanium metal tubes are used together with Muntz copper alloy tube sheets, cathodic protection is usually used to reduce corrosion of copper alloys. When the protection potential is too low, it can cause hydrogen absorption in the effective tube. Analyzing these hydrogen kneeling examples, it can be seen that the hydrogenation of titanium rods in electrolyte solutions generally requires the following three conditions to be met simultaneously:
(1) The pH value of the solution (which may also be local) must be below 3 or above 12.
(2) There must be some mechanism for hydrogen production, such as reducing the corrosion potential (overall or local) of titanium metal in fresh electricity to below the hydrogen evolution potential through uniform corrosion, local corrosion, surface mechanical scratches or pollution, or directly developing hydrogen on the titanium surface due to negative cathodic protection potential.
(3) The ambient temperature must be above 80 ℃, otherwise although there is a mechanism for hydrogen evolution and hydrogen absorption by titanium, hydrogen diffusion is slow below 80 ℃ and hydrogen kneeling is difficult to occur, unless severe tensile stress promotes low-temperature diffusion of hydrogen or hydrogen generated by electrochemical reactions sometimes causes high room partial pressure.
The electromotive force of hydrogen evolution corrosion is much lower than that of oxygen reduction corrosion, and is related to titanium oxidation; The reaction potential of Ti Ti++ne is close. When the supplementation of oxygen (including oxidants) in the solution is slow and difficult to compete with hydrogen evolution, hydrogen evolution type corrosion often dominates. Hydrogen evolution usually occurs under the following conditions.
(1) The standard potential of metals is very negative, such as titanium metal under continuous film removal.
(2) The corrosion potential of metals is lower than the hydrogen evolution potential in electrolytes. When the concentration is high, such as in strong reducing acids.
(3) Due to the formation of complex ions, the concentration of titanium ions in the solution is always kept very low.
