It’s not, no – the codeine is reduced by a kitchen chemistry method analagous to meth production [reduction of allylic alcohol in krokadil v benzylic alcohol in meth]. It also needs to be demethylated – not sure how they accomplish that. The resultant desomorphine is significantly more potent than morphine, and will have a very different feel – clearance rates etc.
The horrific side-effects appear to come from the incompetent process used to prepare krokadil – it’s not an inherent property of the drug to rot your flesh. More a feature of injecting a load of malformed, acidic, dirty shite into your veins on a daily basis.