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. There is another difference between basic blackening and secondary blackening. The main components of blackening liquid are sodium hydroxide and sodium nitrite. When blackening, the temperature resistance is high, and a good surface can be obtained between 135 degrees Celsius and 155 degrees Celsius, but it takes a little longer. In actual operation, attention should be paid to the quality of rust removal and oil removal before blackening and the quality of passivation and oil immersion after blackening. Because of these processes, the quality of blackening often changes. Metal "bluing" liquid medicine adopts alkaline oxidation method or acid oxidation method; Forming an oxide film on the metal surface to prevent the metal surface from being corroded is called "bluing". The oxide film formed by the "bluing" treatment of ferrous metal surface is mainly composed of ferroferric oxide in the outer layer and ferrous oxide in the inner layer.

First, alkaline oxidation of "blue" liquid medicine

1. Formula: sodium nitrate 50 ~ 100g sodium hydroxide 600 ~ 700g sodium nitrite 100 ~ 200g water 1000g.

2. Preparation method: after weighing according to the formula, the ingredients are added in turn under the condition of stirring, dissolved and evenly mixed.

3. Description:

(1) Before the "hair basket" treatment, the metal surface must be cleaned and dried.

(2) The condition of "bluing" treatment of metal devices is related to the carbon content in the metal. The temperature of "bluing" liquid medicine and the treatment time of metal instruments in it can refer to the following table. Carbon content in metal% Working temperature (℃) Treatment time (minutes) Start to end >; 0.7 135- 137 143 10-300.5-0.7 135- 140 15030-50 & lt； 0.4142-145153-15540-60 alloy steel142-145153-/kloc-0.

(3) Regularly analyze the contents of sodium nitrate, sodium nitrite and sodium hydroxide in the solution every other week or so, so as to supplement relevant components in time. Generally, all solutions should be replaced after half a year of use.

(4) After the metal turns blue, it is best to rinse it with hot soapy water for a few minutes, and then rinse it with cold water. Then, rinse it with hot water and blow it.

Second, the "blue" potion is oxidized by acid.

1. Formula: phosphoric acid 3 ~ 10g, calcium nitrate 80 ~ 100g, manganese peroxide/kloc-0 ~15g, water 100g.

2. Preparation method: after weighing according to the formula, under the condition of continuous stirring, phosphoric acid, manganese peroxide and calcium nitrate are added in turn, dissolved and mixed evenly.

3. Description:

(1) Metal equipment can only be "blue" after cleaning and drying.

(2) The protective film obtained by this method is black, mainly composed of calcium phosphate and iron oxides, and its corrosion resistance and mechanical strength are higher than those obtained by alkaline oxidation.

4. The working temperature of "bluing" is 100℃ and the treatment time is 40 ~ 45 minutes. When treating carbon steel, the content of phosphoric acid in the liquid medicine is controlled at 3-5g/L; When treating alloy steel or cast steel, the content of phosphoric acid should be controlled at 5 ~ 10g/L ... The content of phosphoric acid in the liquid medicine should be analyzed regularly.

5. The cleaning method of metal devices after "bluing" treatment is the same as above.

You can also do the experiment yourself:

Generally speaking, steel is easy to rust. If the steel surface is blue, it can greatly enhance the corrosion resistance and prolong the service life. Such as weapons, springs, steel wires, etc. , are often protected in this way. In the mixed solution containing oxidant and caustic soda, after a certain time at a certain temperature, iron reacts to form sodium ferrite (Na2FeO2) and sodium ferrite (Na2Fe2O4), and sodium ferrite and sodium ferrite interact to form an oxide film of iron oxide. The reaction equation is:

3Fe+nano 2+5 NaOH = 3 na 2 FeO 2+H2O+NH3↑

8Fe+3 nano 3+5 NaOH+2H2O = 4 na 2 Fe 2 o 4+3n H3↑

na 2 FeO 2+na 2 Fe 2 o 4+2H2O = fe3o 4+4 NaOH

This oxide film is black or black-blue, with compact structure and strong corrosion resistance.

Supply iron (or nails), tripod, asbestos net, alcohol lamp, measuring cylinder, watch mirror, beaker, tweezers, platform scale, thermometer and sandpaper; Sodium hydroxide, sodium carbonate, water glass, dilute sulfuric acid, sodium nitrate, sodium nitrite, potassium dichromate solution, engine oil.

operate

(1) Take 10g sodium hydroxide, 4g sodium carbonate and 1g sodium silicate, and add 100mL water to prepare degreasing solution; Take 50 g~60g of sodium hydroxide, 10g of sodium nitrate and 3g of sodium nitrite, and add 100mL of water to dissolve them to prepare a bluing solution.

(2) Use sandpaper to smooth the iron sheet, then put it into a beaker filled with 20mL degreasing liquid, heat and boil it for 5 min ~ 10 min, take it out with tweezers, and then rinse it with water.

(3) Immerse the iron sheet whose surface has been degreased in derusting liquid (15% dilute sulfuric acid), heat it (60℃ ~ 80℃) for derusting, and take it out for cleaning after about 2 minutes.

(4) Put the degreased and derusted iron sheet into a beaker containing 20mL bluing solution, heat and boil it for about 20 minutes, then take it out with tweezers, rinse it with water and dry it. You can see that there is a dense black-blue or black oxide film on the iron sheet.

(5) Although the oxide film on the surface after oxidation treatment is relatively dense, there are still tiny pores. In order to improve the corrosion resistance, the iron scale can be put into 3% ~ 5% potassium dichromate solution (90℃ ~ 95℃) for about 105min to fill or passivate the pores, and then put into hot engine oil (105℃ ~10℃).

(6) Soak the blue iron sheet in 2% copper sulfate solution, leave it at room temperature for 20s, then take it out and wash the surface with water. If there is no red contact point, the blue color is qualified.