Excuse me, what's wrong with chemical cooking?

Surface chemical treatment of steel-blackening

At present, there are two kinds of surface oxidation treatment of steel commonly used in factories.

1。 Alkali boiling black

2。 Acid-based cooking

First of all, I will introduce the formula proportion of base boiling black according to what I know.

650 grams of sodium hydroxide

250 grams of sodium nitrite

Water 1L

1. High temperature alkaline oxidation blackening: China Mechanical Information Network /havi6 @ 7x6i6ck.

Alkaline blackening at high temperature (also known as bluing) is the most typical blackening method of steel, which has a history of several decades, the process is relatively mature and the blackening quality is stable. Among the current methods, the appearance, adhesion and corrosion resistance of the film are the most ideal. Therefore, it is still the main method of blackening steel. In this method, a concentrated solution of sodium nitrite and sodium hydroxide is used, and the workpiece to be blackened is put into this solution and boiled at a temperature of about 140℃ (the temperature of blackening solution is slightly different depending on the building materials, generally, it is 138℃ for high carbon steel, 140℃ for medium carbon steel and/or low carbon steel. 16638/66007 The oxide film has good contact with the surface of steel substrate, and has good adhesion and corrosion resistance. China machinery information network

Mechanism of alkaline blackening at high temperature: Many chemical reactions are involved in the process of alkaline blackening at high temperature. First, the following reactions will occur at high temperature.

Yr (c1jw02nano2 = Na2O+n2o3 (1) China machinery information network -b)? iw*]-|*h

The generated N2O3 reacts with hydrogen separated from the solution.

[ 1H:@ bx 02 N2 o 3+6 H2 = 4 NH3+3 O2(2)

) \it(b)E [_"mV0 Oxygen will react with iron in the following series.

2Fe + O2 = 2FeO (3)

f7V.pbf.[04Fe + 3O2 = 2Fe2O3 (4)

，c ' nd 2 _:EHX # e-n7 ~-e03Fe+2o 2 = fe3o 4(5)

In conventional blackening, the temperature of blackening solution should be controlled between 138- 146℃, and the appropriate blackening time is 20-30 min. When blackening, the fastener must be placed in the middle of the groove, and it is not allowed to expose the fastener to the liquid level or near the bottom of the groove. Its formula is: sodium hydroxide (NaOH) 650-700g/L, sodium nitrite (Nano2) 70-75g/L, trisodium phosphate (Na3PO4? 12h2o) 80g/L. The ratio of sodium hydroxide (NaOH) to sodium nitrite (NaNO2) is (8- 12): 1, which is the best. The lower temperature limit and the upper ratio limit are applicable to low carbon steel.

Second, the main defects in the process of blackening

(1) Red-brown rust spots appear on the surface of blackened parts.

(2) The blackened film is light in color.

(3) The surface of the blackened part is red.

(4) The surface of the blackened part is green with uneven color.

Third, the causes of defects and remedial measures

When the content of (1) oxidant (NaNO2 _ 2) is insufficient, the corrosion degree is aggravated, resulting in reddish-brown rust of iron hydroxide (Fe (OH) _ 3). In this case, an appropriate amount of oxidant (NaNO2) can be added, and water can be added to reduce the tank temperature.

(2) When the bath temperature is low and the oxidation is insufficient, the blackening film will become lighter. At this time, according to the test results, the concentration of the bath solution can be appropriately increased, and the temperature of the bath solution can be increased.

(3) When the bath temperature is high and the content of caustic agent (NaOH) is too high, the black surface of the hair piece will turn red. At this time, the tank temperature should be strictly controlled, and the ratio of sodium hydroxide (NaOH) to sodium nitrite (NaNO2) should be strictly controlled, which should not be greater than 12: 1.

The surface of steel parts is black, and some are also called blue.

There are two common blackening methods: traditional alkaline heating blackening and later normal temperature blackening.

However, the blackening process at room temperature is not very effective for low carbon steel.

Alkaline blackening of A3 steel is better.