Como distinguir materiais de aço inoxidável?

How to distinguish stainless steel materials?

1. Nitric acid spot test

A notable 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, high-carbon 420 and 440 steels are slightly corroded during the nitric acid spot test, and non-ferrous metals will be corroded immediately when encountering concentrated nitric acid.

Related Reading: Ferrous vs Non-Ferrous Metals

Diluted nitric acid has strong corrosiveness to carbon steel.

2. Copper Sulfate Spot Test

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

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

Before spot testing, the test area should be thoroughly cleaned of grease or various impurities, and a small area should be polished with a soft abrasive cloth, and then a dropper bottle should be used to drip sodium sulfate solution. copper in the clean area.

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

3. Sulfuric acid test

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

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

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

When 302 and 304 steels are immersed in this hot solution, they are quickly corroded and generate many bubbles, and the sample turns black within a few minutes;

The 316 and 317 steel samples are not corroded or react slowly (no 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 may be more accurate.

4. Magnetic experiment

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

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

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

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