Pickling process is performed to remove rust and scales from surfaces of steel parts, and inhibitors have been in use to prevent excess dissolution of the steel parts. In fact, the inhibitors, in addition for prevention of excess steel dissolution, act to prevent hydrogen embrittlement as well.
Following chemical reactions occur in hydrochloric acid pickling without inhibitor addition.

The hydrochloric acid will dissolve rust and scales on steel surface and forms chlorides but will not generate hydrogen (see Dissolution reaction of steel rust, above). Further into the process, base steel surface is exposed as the rust/scale layer is dissolved and the steel will begin to dissolve (see Dissolution reaction of steel alloy, above), causing hydrogen formation. Here the steel alloy begins to occlude the hydrogen in atomic state and hydrogen embrittlement will result.
The inhibitor prevents this dissolution of the steel alloy thus preventing the formation of hydrogen. The inhibitor prevents the hydrogen formation by selectively adhering to the exposed steel surface to prevent the acid solution from contacting the surface, thus preventing the hydrogen from forming.

As seen above, the use of inhibitors in pickling are effective in hydrogen embrittlement prevention. However, since chemical composition details are unclear for many commercially available inhibitors, it is advisable to test them before actual use.
For sulfuric acid, hydrochloric acid, and phosphate pickling baths, diethylthiourea and dibutylurea are well known.
The chemical compositions of commercial inhibitors are not published, but the following are know to be effective for hydrogen embrittlement suppression, according to some published material.

There is, however, a problem with the commercial inhibitors. Since the chemical composition details are unknown, proper concentration management becomes difficult. At low concentration levels, effective prevention of hydrogen embrittlement cannot be expected, and on the other hand at high concentration levels, the agent that may be carried over may affect the next process negatively.