[Experience learning] A company’s NdFeB electro-galvanizing failure, see their detailed solutions!

Foreword

After our company rolled galvanized the newly developed 45UH magnetic material, there were many macroscopic pits, roughness, and serious missing corners on the surface of the coating. Can not meet customer requirements. For this reason, all processes were checked and analyzed, and the fault was eliminated.

Process and product requirements

01. Product brand and size

NdFeB 45UH, 32.5mmx22.0mmx16.0mm

02. Electroplating process

Incoming slices—vibration finishing (chamfering)—washing (2 channels)—pickling (3% nitric acid)—washing (2 channels)—activation—washing (2 channels)—barrel galvanizing—recycling (2 Channel)—washing (2 channels)—polishing—washing (2 channels)—passivation—washing (2 channels)—hot water washing—blowing dry.

03. Galvanizing process

The formulation of the plating solution 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 of 1.0~1.5A/dm2, PH value of 5.4, drum speed of 6r/min, number of products with 1L steel ball, 40 pieces, normal temperature, time of 60min, continuous filtration.

04. Qualified product performance

The thickness of the coating is about 10μm, and it is resistant to neutral salt spray corrosion for 72h. The surface is smooth and unskinned, blue and white and bright in color, with normal dimensional tolerances and no missing corners.

05. Laboratory inspection and analysis

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.

Symptoms

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.

Fault analysis and handling

After analyzing and testing the composition of the tank liquid and the performance of the finished product, it is determined that there is no fault in the tank liquid barrier treatment and the post-treatment passivation solution, 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 changing water frequently to protect the workpiece from corrosion during vibration chamfering. However, high-grade 45UH magnetic materials rely on more rich microstructures. Nd phase and rare earth elements to ensure its high coercivity, which also greatly improves the corrosion and brittleness of the material.

After checking after chamfering, the material is indeed corroded and impacted in the chamfer, 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 also decompose in the chamfer to generate weak organic acid, which will further increase the corrosion of the material.

01. Small pit treatment

Considering the high cost of commercial chamfering fluid, after orthogonal optimization, a self-prepared chamfering fluid containing 6-8g/L sodium benzoate and 2-3ml/L triethanolamine is used for vibration light finishing chamfering.

As a commonly used food preservative, sodium benzoate has weak alkalinity and lipophilicity. Its hydrolysis is weakly alkaline, and the reaction equation is C6H5COONa+H2O→C6H5COOH+NaOH

At the same time, triethanolamine may react with a small amount of organic glue or more oleic acid left by the slicing process under weak alkaline and vibration chamfering conditions to form triethanolamine lipid compounds, which are attached to the metal surface as a good Corrosion inhibitor, protect the workpiece from being oxidized and corroded.

At the same time, these lipid compounds are easier to remove under ultrasonic cleaning, and they also have a certain degreasing effect.

02, missing corner treatment

Vibrating light chamfering 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. It is an important means to improve the corrosion resistance of the coating.

After multiple sets of debugging, the brown corundum stone with a size of about 1/4 of the workpiece size is selected as the abrasive, and the number of round abrasive stones is increased. Ball: rhombus: square = 4:2:1, improving the work The mass ratio is 1:3, the self-made chamfering liquid is added to the liquid level above 5-8cm, the vibration frequency is increased to 36Hz, and the vibrating light chamfering is 2.5h, then the workpiece is washed with water.

03. Processing results

The workpiece obtained under the new chamfering process conditions is subjected to pretreatment, electroplating, passivation, etc. according to the original process to obtain a 45UH finished product of magnetic material, the color is blue and white, the appearance is flat, and there are no small pits under the observation of a 20x magnifying glass. , The corner arc is qualified, the missing corner is reduced and the missing corner size is also in the normal range. Performances such as neutral salt spray are also qualified.