The stainless steel kettle at home always has black stuff on it, what's going on? Is it poisonous?

I don’t know if you have ever encountered this situation: some stainless steel containers look very clean, even with a metallic luster, but when you wipe them off with paper, black stuff appears.

Image source: Screenshot of social platform

Can such stainless steel items still be used? Especially stainless steel water cups, which are used directly to hold drinking water, are they really safe?

What is the black stuff that comes off?

There are several possibilities for the black stuff on stainless steel containers.

1

New stainless steel appliances

If you find black stuff in your newly bought stainless steel pot or stainless steel cup, it may be wax and metal debris left over after polishing the stainless steel container.

ASTRO PAK (a company specializing in the cleaning of precision devices for aviation and military use) once analyzed the black residual particles on the surface of stainless steel and found that the residues mainly included two major types of substances: organic fatty acids and their analogues, and metal debris such as iron, chromium, and aluminum.

It is difficult to clean these things with water directly. ASTRO PAK's analysis report gives some suggestions. Among them, the most suitable one for ordinary people is to use alkaline detergent (such as dishwashing liquid) to clean them, and wipe them continuously while cleaning . Although it is a bit "labor-intensive", it is indeed effective.

In addition, there is a method of using flour to clean the surface that is circulated on the Internet. It can be simply understood as taking a stainless steel basin or pot and mixing flour, and using the dough to stick off the residue on the surface. Although there is no paper to study this, it sounds like a relatively feasible method. However, it is recommended to use detergent to clean and wipe it several times after sticking it on to "consolidate it."

2

Stainless steel appliances that have been used for a long time

Some stainless steel appliances may not have black substances at the beginning, but after long-term use, some black substances will appear. These substances are no longer polishes, they may be the "protective film" of the stainless steel is broken. The black substances may be metal oxides of iron, aluminum or chromium. Don't worry, these black substances usually do not cause harm to human health.

Chromium trioxide powder, black with a green tint, Image source: Wikipedia

If there are only a few black substances, you can try to clean them with white vinegar, but be careful not to scratch them with items such as steel wool to avoid causing greater damage to the protective film on the surface of the cup. If black substances appear frequently, it is recommended to replace stainless steel utensils, because frequent appearance may mean that the protective layer has been severely damaged.

Stainless steel is also a very large family. It refers to alloy steel that can remain rust-free in air, water, and special acidic and alkaline environments. The way most stainless steels remain "rust-free" is to form a dense protective film on the surface of the steel, thereby protecting the internal iron from oxidation. Take 304 stainless steel, which is often heard in life, as an example. The secret of their protection from rusting is related to the chromium element in the stainless steel material. The chromium content of 304 stainless steel is not low, between 18% and 20%. Many people do not have a good impression of chromium, thinking that it is a toxic heavy metal. Indeed, hexavalent chromium is toxic and is a Class I carcinogen (clearly carcinogenic) recognized by the International Agency for Research on Cancer (IARC). But it does not mean that chromium is useless. For example, trivalent chromium is an essential trace element for the human body. As long as the intake does not exceed the standard, it is beneficial to the human body. In stainless steel, after chromium comes into contact with air, it will quickly form a dense oxide film (chromium trioxide) film, which can prevent the air from continuing to oxidize the metal inside, thus making the stainless steel "rust-free".

What are 304, 316 stainless steel?

If you choose a stainless steel water cup or a container for food, you will find that most of the stainless steel is 304 stainless steel, or 316 stainless steel, and occasionally you will see 430 stainless steel. Do these stainless steel models have any special meaning?

In fact, 304, 316, and 430 are the numbers given to these stainless steels by the American Iron and Steel Institute. Different countries, regions, and organizations have different numbers for stainless steel.

For example, in the UNS numbering system widely accepted in North America, 304 stainless steel is also called S30400. Our country's "Method for Representing Steel Product Grades" (GB/T221-2008) numbers stainless steel according to its carbon content and other metal content. 304 stainless steel is called 06Cr19Ni10.

06 means that its carbon content does not exceed 0.08%, Cr19 and Ni10 represent the chromium and nickel contents respectively.

The meaning of stainless steel grades, Image source: Reference 3

Therefore, don’t be surprised if you see the stainless steel material is 06Cr19Ni10 on the product certificate. It is the commonly known 304 stainless steel.

Another commonly heard stainless steel is 316 stainless steel, which generally refers to 316L stainless steel, and the corresponding grade is 022Cr17Ni12Mo2 (the old grade is 00Cr17Ni14Mo2). Mo means that it contains more molybdenum, which also improves the corrosion resistance of stainless steel, especially in chlorine-containing conditions such as seawater and salt water. Its corrosion resistance is better than that of 304 stainless steel.

So, is it best to buy 316L stainless steel for stainless steel that comes into contact with food?

Which stainless steels can be used as food containers?

It is necessary to explain here that 304 stainless steel and 316L stainless steel have good corrosion resistance, so they are the most common stainless steel food container materials. But it does not mean that 304 stainless steel and 316L stainless steel are equivalent to "food grade stainless steel", they are concepts of different dimensions.

Our country's "Food Safety Standard Metal Materials and Products for Food Contact" (GB 4806.9-2023) has clear requirements for impurities in stainless steel materials and the migration of alloy elements in the materials. If the stainless steel material can pass the tests specified in this standard, it can be used as a food material, which is also called "food grade".

For example, some places say that 430 stainless steel is not food grade stainless steel. In fact, this statement is not rigorous. It cannot be said that all 430 stainless steel cannot be used for food containers. The key is to see whether this batch of materials can pass the test in GB 4806.9-2023.

If you see that the steamer and cutlery in your home are made of 430 stainless steel, don't worry. As long as they are produced by a regular manufacturer and have a product certificate, it means that they have passed the test and will not cause harm to your health.

On the other hand, if a batch of 304 stainless steel fails the alloy element migration test, it cannot be used as food containers.

Therefore, when buying stainless steel tableware or stainless steel water cups, it is recommended that you check whether its implementation standard is GB 4806.9-2023. As long as it is this standard and has a certificate of conformity, you can use it with confidence.

Of course, if you have to compare 304, 316L, and 430 stainless steel, in terms of corrosion resistance, the ranking is 316L, 304, and 430. But the price of 316L stainless steel will be relatively higher, so you can consider buying it according to your needs and wallet situation.

References

[1]https://astropak.com/wp-content/uploads/Black-Residue-Remediation-rev-1.pdf

[2] Stainless Steel Cold-rolled Sheet and Strip (GB/T 3280-2015)

[3] “Methods for indicating steel product grades” (GB/T 221-2008)

[4] Food Safety Standard Metal Materials and Products for Food Contact (GB 4806.9-2023)

Planning and production

Author: Science Scraps Popular Science Writer

Review丨Sun Mingxuan, Professor of Shanghai University of Engineering Science

Planning丨Fu Sijia

Editor: Fu Sijia

Proofread by Xu Lai and Lin Lin