Zinc alloy surface gray protective layer and preparation method thereof

The invention discloses a zinc alloy surface gray protective layer. A treating fluid for preparing the protective layer comprises the following raw materials: 20-50g/L of phosphoric acid; 20-50g/L of saturated dicarboxylic acid; 2-5g/L of ammonium nitrate; 0.1-5 g/L of a copper-containing compound; and the balance of water. The invention also provides a preparation method of the zinc alloy surface gray protective layer, which comprises the following steps: carrying out surface oil removal and acid pickling treatment on the zinc alloy, and then cleaning the zinc alloy with clear water; preparing a treating fluid according to a formula comprising 20-50g/L of phosphoric acid, 20-50g/L of saturated dicarboxylic acid, 2-5g/L of ammonium nitrate, 0.1-5g/L of a copper-containing compound and the balance of water; and putting the cleaned zinc alloy into the prepared treating fluid, treating for 15 seconds to 15 minutes, then taking out, cleaning and drying to obtain the gray protective layer. After the titanium alloy