Do Ferroxyl test.

As you are seeing Rusty appearance after Passivation ----------- Step1 :- Ensure your 17-4 contains significant Sulphur or not Step 2 : You have to ensure surface is free from embedded Iron particles by Standard Ferroxyl Test. Step 3 : If surface indicates Free Iron first you have to clean and degrease surface from all organic contaminants by Grit Blasting and Degreasing for cast surface Step 4 : You have to do Acid Pickling by HNO3 ( 20%) + HF ( 2%) mix Step 5a :- If your material is NON-SULPHUR BEARING then you do Passivation by 50%by volume HNO3 at 50C - 60C for 40 m inutes maximum. Step 5b : If your material is SULPHUR BEARING then you do Passivation by A-A-A Method ( Alkaline - Acid - Alkaline Method ) as below -- After Pickling soak the part in 5% solution of NaOH at 75 - 80C for 30 minutes ---> Rinse the entire part by DM Water ( < 50 ppm chlorides )spray ---> Immerse the part fully for 30 minutes in a 20% HNO3 solution containing 22 gm / Litre Sodium Dichromate at 50 - 60C ---> Remove from Acid Bath and Water Rinse ---> Again immerse in 5% solution of NaOH at 75 - 80C for 30 minutes --> Passivation is complete. Hope this will work. regards