Aluminium becomes naturally dull when washed in a dishwasher.
China or glass items, for example, casserole dishes may show dark (black or grey) discolouration.
This is not a fault with your dishwasher, please follow the steps below to avoid this problem.
Usually the result of:
Natural oxidation of aluminium
Cutlery being allowed to scrape the surface, for example during mixing
Cause & Prevention
We usually find that discolouration of metal is simply natural oxidation. This is especially so with aluminium items which will become naturally dull over time if washed in a dishwasher.
Black marks, often found on china or glass is invariably the result of scraping metal cutlery or utensils against the surface.
There is no treatment for these issues and the damage is irreversible.
Do not wash aluminium items in the dishwasher.
Avoid scraping with metal implements.
Purchase cookware that is clearly marked as being dishwasher safe.
Pay attention to specialist items, such as cooking knifes, as many do state that they are unsuitable for washing in a dishwasher or that it is not advised.