Generally speaking, the pins of network transformers are copper-clad steel, which rusts in the air for a short time, which is called oxidation. The treatment method that can be adopted: soak the water in hydrochloric acid for 30 minutes, and then tin-plate with water-soluble flux! According to the degree of oxidation, hydrochloric acid can be concentrated! If the stitches made of phosphor bronze are oxidized, this method can't be used! See the next chapter for the treatment method. .........