6.2L/24CM Stockpot with Lid

第一次煲完湯後,清洗完後已經出現彩虹紋

Have been using Meyer products for many years. Newly bought this 7.6L stainless steel pot in Jan 2022. Good price, good look. Have followed caring instructions before use. After first time usage, hundreds if not thousand black spots at base of pot. Can't remove by any cleaning method for stainless steel.
Then I make use the replacement service for a new one. Good service. However not much information about grade of steel used (e.g. 200, 304 etc,). Just mentioned it is Japanese steel, yet no further information on grade level. Not available on Meyer website neither. From price point, I guess it is Japanese steel equivalent to 304 or below.
After using the replacement pot to prepare the same food except to put in salt after water boils, still found hundred dark spots. Not easy to remove. Looks like corrosion by the salt (It is wisdom from internet after some study: black spots are due to corrosion by chloride, called pitting. Usually happening to 304 or lower grade steel).
I wonder if it is due to production batch issue (that I should go and replace again) or it is a product grade issue (that a lessened durability to be expected ).
In order to work out what to do next, I have tried to cook the same dish again with my another 10-year old Meyer stainless steel pot, a cheaper and smaller pot. Only few spots appeared and can be removed easily!!! It is without spots after 10 years.
I wonder if Meyer has changed its steel materials at some point of time in these years. This smaller pot has the word "Durable" casted at bottom but the new pot doesn't. Is this an indication of grade of steel being used? Hope Meyer give more product information for a proper user expectation and usage.

Stainless Steel Stockpot with Lid