How to find the cause of small pits in the NdFeB galvanized coating?

An enterprise is rolling a newly developed 45UH magnetic materialgalvanized, there are many macroscopic small pits, roughness and unevenness on the surface of the coating, and the missing corners are more serious, which cannot reach customer’s request. For this reason, all processes were checked and analyzed, and the fault was eliminated.

1 Process and product requirements

(1) Product brand and size NdFeB 45UH, 32.5mmx22.0mmx16.0mm

(2) Slicing incoming material for electroplating process-vibration finishing (chamfering)-washing (2 passes)-pickling (3% nitric acid)-washing (2 passes)-activation-washing (2 passes)-barrel plating Zinc-recovery (2 courses)-washing (2 courses)-polishing-washing (2 courses) -> passivation-washing (2 courses)-hot water washing-drying.

(3) The formula of the plating solution for the galvanizing process is 50-60g/L zinc chloride, 180-220g/L potassium chloride, 30-40g/L boric acid, appropriate amount of potassium salt zinc plating softener and brightener. The process conditions are: cathode current density 1.0~1.5A/dm2, PH value 5.4, drum speed 6r/min, with rolling steel ball 1L, product quantity 40, temperature and normal temperature, time 60min, continuous filtration.

(4) Qualified product performance The coating thickness is about 10μm, neutral salt spray corrosion resistance is 72h, the surface is flat without skin, blue and white bright color, the dimensional tolerance is normal, and there is no missing corner.

(5) The content of each component of the tank liquid is within the normal range, the grade and size are normal, and the thickness and color are basically normal.

2 Failure phenomenon

Using the above process, the surface of the finished material obtained by barrel plating is rough and uneven, and there are many dense small pits, which are visible to the naked eye, especially the corners, the appearance is ugly, the salt spray is bad, and it is directly scrapped. At the same time, there are many Missing corners. High-grade UH45 magnetic materials are expensive and have serious economic losses.

3 Failure analysis and treatment

After analyzing and testing the composition of the bath liquid and the performance of the finished product, it is determined that there is no fault in the bath liquid and the post-treatment passivation liquid, and there is no change in the pre-treatment pickling and activation process. Due to the high cost of traditional commercial chamfering fluids, our factory often uses simple methods of frequent water changes to protect the workpiece from corrosion during vibration chamfering. However, the high-grade 45UH magnetic material relies on more Nd-rich phases in the microstructure. Rare earth elements to ensure its high coercivity, which also greatly improves the corrosion and brittleness of the material. After the chamfering check, the material is indeed corroded and impacted in the chamfering, resulting in small pits and missing corners. At the same time, in the slicing process before the incoming material, if the glue that must be used is not cleaned, it will decompose in the chamfer to generate weak organic acid, which will further increase the corrosion of the material

(1) Small pit treatment considering the high cost of commercial chamfering fluid, after orthogonal optimization, vibration is carried out with a self-prepared chamfering fluid containing 6~8g/L sodium benzoate and 2~3mL/L triethanolamine Finish chamfering treatment. As a commonly used food preservative, sodium benzoate has weak alkalinity and lipophilicity. Its hydrolysis is weakly alkaline. The reaction equation is C6H5C00Na+H2O—C6H5COOH+NaOH. At the same time, triethanolamine may react with a small amount of organic glue left by the slicing process or more oleic acid under weakly alkaline and vibration chamfering conditions to form three Ethanolamine lipid compounds, attached to the metal surface as a good corrosion inhibitor, protect the workpiece from oxidation and corrosion. At the same time, these lipid compounds are easier to remove under ultrasonic cleaning, and they also have a certain degreasing effect

(2) The chamfering of the chamfered corner processing vibration light can not only reduce the roughness of the material, but also make the corners form an arc to reduce the priority corrosion of the corners, which is an important means to improve the corrosion resistance of the coating. After multiple sets of debugging, brown corundum stone with a size of about 1/4 of the size of the workpiece is selected as the abrasive, and the number of round abrasive stones is increased. Ball: rhombus: square = 4:2:1 to increase the mass ratio of the workpiece to the abrasive to 1. :3. Add the self-prepared chamfering liquid to the liquid surface above 5~8cm, increase the vibration frequency to 36Hz, vibrate the chamfering for 2.5 hours, wash out the workpiece.

(3) Processing result The workpiece obtained under the new chamfering process conditions will be pre-processed, electroplated, passivated according to the original process to obtain a 45UH finished product of magnetic material, the color is blue and white, and the appearance is flat. It is observed under a 20x magnifying glass. There are also no small pits, the corner arcs are qualified, the missing corners are reduced and the missing corner size is within the normal range. Performances such as neutral salt spray are also qualified.