How to Distinguish Stainless Steel Materials?

1. Nitric acid point test

A remarkable feature of stainless steel is its inherent corrosion resistance to concentrated nitric acid and dilute nitric acid.

This property allows it to be easily distinguished from most other metals or alloys.

However, the high carbon 420 and 440 steels are slightly corroded during the nitric acid point test, and nonferrous metals will be corroded immediately when encountering concentrated nitric acid.

Related reading: Ferrous vs Non-ferrous Metals

Dilute nitric acid has strong corrosivity to carbon steel.

2. Copper sulfate point test

This is the easiest way to quickly distinguish between ordinary carbon steel and all types of stainless steel.

The concentration of copper sulfate solution used is 5-10%.

Before the spot test, the test area shall be thoroughly cleaned of grease or various impurities, and a small area shall be polished with a soft abrasive cloth, and then a dropping bottle shall be used to drip copper sulfate solution into the cleaned area.

Ordinary carbon steel or iron will form a layer of surface metal copper in a few seconds, while the surface of stainless steel will not produce copper precipitation or show the color of copper.

3. Sulfuric acid test

The sulfuric acid immersion test can distinguish 302 and 304 from 316 and 317.

The cut edge of the sample shall be finely ground, and then cleaned and passivated for half an hour in nitric acid (specific gravity 1.42) with a volume concentration of 20~30% and a temperature of 60~66 ° C.

The volume concentration of the sulfuric acid test solution is 10%, and it is heated to 71 ℃.

When 302 and 304 steel are immersed in this hot solution, they are rapidly corroded and generate a lot of bubbles, and the sample turns black within a few minutes;

The 316 and 317 steel samples are not corroded or react slowly (without bubbles), and the samples do not change color within 10-15 minutes.

If the sample with known composition is tested at the same time for approximate comparison, the test can be more accurate.

4. Magnetic experiment

The simplest way to distinguish between annealed austenitic stainless steel and ferritic stainless steel.

Austenitic stainless steel is non-magnetic steel (but it does not mean that it has no magnetism), but it will have slight magnetism after cold working under heavy pressure;

Pure chromium steel and low alloy steel are strong magnetic steel.

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