First, the requirements of conventional blackening and formula
In the conventional blackening, the temperature of the blackening liquid should be controlled between 138-146°C, and the blackening time should be 20-30 min. When blackening, the fasteners must be placed in the middle of the bath and the fasteners must not be exposed at the liquid level or near the bottom of the tank. 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) and sodium nitrite (NaNO2) is preferably (8-12):1. Low carbon steel uses the lower limit of temperature, and the upper limit of the ratio is appropriate.
Second, the main defects in the process of blackening
(1) Red-brown rust on the surface of the blackened part.
(2) The color of the blackened film is pale.
(3) Redness of the blackened parts.
(4) The surface of the blackened part is green and the color is uneven.
Third, the cause of the defect analysis and remedy
(1) When the content of oxidant (NaNO2) is insufficient, the degree of corrosion increases, and iron hydroxide (Fe(OH)3) reddish brown rust occurs. In this case, add an appropriate amount of oxidant (NaNO2) and add water to lower the bath temperature.
(2) When the bath temperature is low and the oxidation is insufficient, the blackening film will fade in color. At this time, according to the test results, the concentration of the bath solution can be appropriately increased, and the bath temperature can be increased.
(3) When the bath temperature is too high and the content of corrosive agent (NaOH) is too much, redness may occur on the black surface of the transmitter. At this time, the tank temperature should be strictly controlled, and the ratio of sodium hydroxide (NaOH) and sodium nitrite (NaNO2) should be strictly controlled. The ratio cannot be greater than 12:1.
(4) When the bath temperature is low, the loading amount is too high, and the bath concentration is low, greening will occur on the surface of the blackened parts, and the color will be uneven. At this time, the bath temperature and bath concentration should be increased