Hardness is a major contributor to hydrogen embrittlement. Harder, stronger materials are more susceptible to failure than weaker, softer ones. In general, if the hardness of the component is less than 35 HRC, there will probably be little difficulty with hydrogen embrittlement. However, if the component has hardness above 40 HRC, problems are more likely to occur.
Hydrogen embrittlement can occur during various manufacturing operations or operational use – anywhere that the metal comes into contact with atomic or molecular hydrogen. Processes that can lead to this include cathodic protection, phosphating, pickling, and electroplating.
Other mechanisms of introduction of hydrogen into metal are galvanic corrosion, chemical reactions of metal with acids, or with other chemicals (notably hydrogen sulphide in sulphide stress cracking, or SSC, a process of importance for the oil and gas industries).
Higher acid concentrations and long exposure times will increase the hydrogen concentration in the component material, thus increasing the likelihood of hydrogen embrittlement.
The Blackfast process
The cleaner, Blackfast 716, is an alkaline solution and should therefore not be capable of producing hydrogen embrittlement.
The conditioner, Blackfast 551, is acidic (pH 2.5-3) and contains small amounts of phosphoric and hydrofluoric acid (less than 0.1% in the working solution) and immersion time is no more than 2 minutes.
The blacking solution, Blackfast 181, is also acidic (pH 3-3.5) and contains phosphoric acid and potassium bifluoride (less than 1% in the working solution) and immersion time is only 1 minute.
It is concluded that the blackfast process contains very low amounts of acid and the immersion times are minimal which should mean that the process is unlikely to cause hydrogen embrittlement.
Coating porosity also has an impact on hydrogen concentration. Electroplated coatings are dense enough to “trap” or seal hydrogen in the base material. Once the hydrogen is sealed in the component it is more likely to produce an embrittlement failure. Mechanical coatings are more porous (less dense). Therefore, any hydrogen in the base material of a mechanically coated component will have a better opportunity to escape.
The Blackfast process
Is not a dense coating, rather a surface conversion, and it is specifically designed to produce a porous crystal lattice which will allow oils and waxes to be absorbed into it.
The Blackfast process is therefore unlikely to be capable of trapping or sealing hydrogen in the component.