This is quite common and pretty harmless. The scratches you see don't go very deep, nor are they very wide. My All-Clad saute pan is nearing 10 years old and has a ton of micro-scratches on the interior. It still performs beautifully.
That said, the scratches can grab onto proteins and cause sticking. However, this is simple to prevent with both oil and proper pan preheating.
When a pan is preheated properly the metal expands, essentially closing all of the micro-scratches. This prevents the proteins from grabbing onto them and getting stuck. You obviously need oil/fat to assist with this as well.
To properly heat a pan to the appropriate temperature I suggest using the water drop method. If you put a cold pan on heat and drip a drop of water onto it, the water will sit there for several seconds then boil away. As the pan gets warmer this will happen more quickly, fizzling away in a second or so. Once the scratches start to close something weird happens.
First, the drop of water will break into a few mini drops which scoot around the pan as they evaporate. This is a sign that you are almost there. When the drop of water stays whole (mostly) and scoots around the pan like a mercury ball, this is the perfect temperature. I the water instantly vaporizes on contact, you've gone way too far and need to let the pan cool down. At this point you should add your oil/fat, swirl it around, and immediately add your food. (Make sure the mercury ball of water is gone before adding oil).
Also note that the mercury-ball phase is definitely too hot for unclarified butter, and may be too hot for some extra-virgin olive oils. They may instantly smoke upon adding.
Again, it's important to have your oil and ingredients in place (mise en place) before you start. It's quite easy to skyrocket past the mercury-ball phase if you have to open your oil, pour, and then season your ingredients.
Coated (e. g. enamel, PTFE, ceramic)
I can't answer in general, but that one's easy. Sudden thermal shock causes strain in a material by unequal expansion, either in the same material by high thermal gradients, or in interfaces between materials with different coefficients of thermal expansion. The strain in this case (two different materials) can be very high. If the material in question is not elastic (e.g. enamel + ceramic; I would think PTFE is different, but I'm not sure), then the bonds between the coating and the metal would be severely strained and it would likely crack and chip.
I can tell you from personal experience that I have actually used this to my advantage:
In the spring, I produce a small quantity of maple syrup by boiling sap in an uncoated stainless steel pan. On rare occasions, accompanied by the release of many expletives, I have let the syrup boil down too far, at which point it burns and seems to coat the bottom of the pan with a thin but hard and very resilient layer of carbon black. The trick to removing this stuff is to get some kind of stress crack started, e.g. by scrubbing w/ steel wool or a copper pad, and then what I do is I put the pan on the stove for a while to let it heat up hot (but not red hot), and then bring it over to the sink and spray cold water on the inside pan bottom where the carbon black has stuck to. After a few times, the carbon black will start to flake off and then it becomes easier to remove by a combination of abrasion and thermal shock. (The two pans I've done this on have been fine; both are stainless steel with a thick (>8mm) bottom, and I've put them through at least 30 or 40 thermal cycles of this type.)
edit re: general topic:
Wikipedia says this:
The robustness of a material to thermal shock is characterized with the thermal shock parameter:
where
- k is thermal conductivity,
- σT is maximal tension the material can resist,
- α is the thermal expansion coefficient
- E is the Young's modulus, and
- ν is the Poisson ratio.
Higher thermal conductivity means it's more difficult to get a large thermal gradient across the material (less prone to shock); higher thermal expansion means more strain (more prone to shock), and higher Young's modulus means more stress for a given strain (more prone to shock).
So theoretically you could compare the different materials. (exercise for the reader ;) Most likely copper would be more resilient than the other metals, because of its higher thermal conductivity and higher ductility.
Thermal conductivity k: Copper = 401, Aluminum alloys = 120-180, stainless steel = 12-45 (units = W/m*K)
σT: no idea:
Coefficient of thermal expansion α: Copper = 17, Aluminum = 23, iron = 11.1, stainless steel = 17.3 (units = 10−6/°C)
Young's modulus E: Copper = 117, Aluminum = 69, iron/steel = around 200 (units = GPa)
Poisson's ratio ν: Copper/stainless steel/aluminum are all around 0.3-0.33, cast iron = 0.21-0.26
So stainless steel is worse than aluminum or copper (much lower thermal conductivity, higher Young's modulus).
Best Answer
Cos, I am not a metallurgist, so I cannot vouch for the authority of this site, but it is a starting point for US rules. https://www.marlinwire.com/blog/food-safe-metals-for-sheet-metal-wire-forms. They list a couple of grades of steel, namely Grade 304 and Grade 316 stainless that are approved, and some general guides as to what metals are inspected for (FDA):
In general though, stock metals would be rated for reactivity, finish, a number of factors, and are assigned industry names or titles, and by grade those are declared food grade or not. Non-food grade metals can be treated as food grade for storage by putting a protective coating, but that would not make them food grade for cooking, only for storage. Cast Iron tends to be the opposite, approved for cooking, but not for storage as it is reactive to acids and porous.
Plastics are the same way, some are way too porous and absorbing to be used for food. We tend to assume that if it is being sold as a kitchen item it is food safe, and usually that is likely true. I only think it is an issue if we are attempting to adapt something which was not manufactured for food use, though there have been far too many instances of imported items into the US and EU that have blatantly violated manufacturing rules and skipped merrily past inspections making many of us edgy of certain countries of origin.
Edit from material provided from the NSF Food Equipment Materials Standard The attached document from the NSF provides clear criteria and test methodology for features like 'cleanability'(section 5) 'corrosion resistance' (6) and then in section 7 lays out requirements for materials.
While the term 'food grade' is never used this certainly looks like a standard that would be 'commonly referred to' as 'food grade'