Unfortunately this situation can and will happen when the technician isn’t properly trained on the pH scale, and the effects that pH and chemical properties have on specific materials.
Acids are a “specialty cleaner” that are very effective when used in a “controlled application “ and only after fully understanding the type of acid and the exact ID of the material it will be used on.
In this unfortunate situation the acid used was hydrochloric (HCI) and not phosphoric.
Hydrochloric acid is extremely aggressive and will attack / dissolve silica.
Phosphoric acids don’t dissolve silica but are very effective when used on lime / calcium based materials.
Prochem “Tile & Grout Restoration Cleaner” is a phosphoric acid and is designed to chemically dissolve calcium deposits and chemically react to lime based material like grout.
It can also be very effective on rust removal.
The phosphoric based grout cleaner / brighteners work through a chemical reaction with the lime in the grout which microscopically etches the grout exposing new grout and thus making it look cleaner and brighter.
If a phosphoric acid was used in this situation, the damage to the glaze (etching) wouldn’t have occurred.
In this situation a cheap toilet bowl cleaner was used, which almost all toilet bowl cleaners are hydrochloric based .
Using Hydrochloric acid in this case attacked / dissolved the glaze (silica) on the “through body porcelain” tiles around the edges .
It will also degrade / attack the grout in a much more aggressive manner than phosphoric acid cleanser, and can turn the grout into powder.
Unfortunately, this damage can’t be fixed in this case and the tiles should be replaced.
Lesson learned …..NEVER use a cleaning product without knowing the chemical properties (MSDS) and how it will affect the material it’s to be used on.
My rule of thumb is this……”start conservative” and move more aggressive in your approach, whether in pH ( acids vs alkaline) , dilution and or physical agitation / abrasiveness.
Also remember when using acids it is always best practice to “neutralize “ the area after using acids with an alkaline solution.