Coolant leak

[dom14]

+1 for housing corrosion. All too common
People think the coolant is OK because it is green, so they don't change it. (kind of like how most tend not to change brake fluid) Once it turns salt it allows electrolytic corrosion to occur within the cooling system.
First bits to go seem to be steel components like welch plugs, steel coolant pipes and yes - steel thermostat housings. Next is the alloy parts that turn to crumbly chalk.

Certainly true mate. BTW, the aluminium thermostat housings are no better than mild steel ones when it comes to corrosion from the acidity of the coolant.(as you can see from the above picture I've posted)
High quality corrosion resistant ones are pretty damn expensive.
I think, checking coolant PH level and replacing it yearly or so should be part of regular maintenance.