Cavitation will give an uneven, pitted surface, in cases somewhat similar to casting faults failure cracking, but with more and deeper local pittings. Abrasion will grind and smooth out surfaces. With crack opening fo a ballvalve, even hairline crackings in sealing surfaces, the pressure drop will be large enough against lower pressures on downstream side, to cause cavitation.
(You can 'forget' temperature. If you look at the steam table, look at extreme low-pressure points for 'boiling', and dont forget that it is the collapse of steam-bubbels when pressure increases again by expanding that gives the high-pressure implosion shock needle-points, said to be some 1000 bars (or more!), that is the definition of cavitation)
I have several times seen damages in pipelines for steam and feeding water ballvalves, quite similar to the picture here, all caused intially by hairline crackings by damages and/or not 100% closed valves. 180 degree damages indicate partially opened valve or partial sealing damage.
As there is limited reports on damages on other parts, it is difficult to be 100% sure of initial cause.
I believe personally in initial hair cracking and cavitation, later worn down a bit over prolonged exposure to by then a larger opening (to large to give sufficient low pressure to cause cavitation) to restricted flow, maybee even started or assisted by material faults or weak materials before, after or during leakage and flow damage.
Maybe cutting, grinding and microscopic material analysis of samples near damaged area could assist in testing material quality, to rule out or include this.