Pickling and Passivation for Stainless Steel Wedge Wire Screens

Every stainless steel wedge wire screen undergoes thousands of resistance welds during manufacturing. Each weld generates heat that depletes chromium from the surface layer and creates visible heat tint (temper colors ranging from straw yellow to dark blue). This chromium-depleted zone is vulnerable to corrosion -- precisely where the screen needs to be strongest. Pickling: Removing the Damage Pickling is an acid bath treatment that dissolves the heat-affected zone, removing heat tint, oxide scale, weld discoloration, and free iron contamination from the surface. The process uses a mixture of nitric acid (HNO3) and hydrofluoric acid (HF), or proprietary pickling pastes for localized treatment. The acid dissolves approximately 0.02-0.05 mm of surface material, eliminating the chromium-depleted layer entirely. What remains is fresh, undamaged stainless steel with its full chromium content available for passivation. Pickling parameters for wedge wire screens: 1. Temperature: 20-60 degrees C (higher temperature = faster action) 2. Duration: 15-60 minutes depending on heat tint severity 3. Concentration: 15-25% HNO3 + 1-5% HF (typical bath) 4. Rinsing: Thorough freshwater rinse immediately after treatment Passivation: Rebuilding the Shield Passivation is a separate chemical treatment that follows pickling. It uses a milder acid bath (citric acid or dilute nitric acid without HF) to dissolve any remaining free iron on the surface and promote the formation of a uniform, continuous chromium oxide (Cr2O3) layer. This chromium oxide layer -- approximately 1-5 nanometers thick -- is the invisible shield that gives stainless steel its corrosion resistance. The layer is self-healing: if scratched, atmospheric oxygen reacts with surface chromium to rebuild the oxide film within hours. Passivation parameters: 1. Citric acid method: 4-10% citric acid at 50-70 degrees C for 30-60 minutes 2. Nitric acid method: 20-50% HNO3 at 20-50 degrees C for 30-60 minutes 3. Verification: Water break test (uniform wetting = good passivation), copper sulfate test (no copper deposits = no free iron) When Is Each Treatment Required? Pickling is required after manufacturing for any screen that will operate in corrosive environments (chlorides, acids, seawater), food-contact applications (FDA, ASME BPE), or pharmaceutical and bioprocess applications. Screens for fresh water or dry screening in non-corrosive environments may not require pickling. Passivation is recommended for all stainless steel screens. Even screens that do not require pickling benefit from passivation to ensure maximum corrosion resistance from day one. For food and pharmaceutical applications, both pickling AND passivation are typically mandatory, followed by electropolishing for the smoothest possible surface finish. Specifying Surface Treatment When requesting a quote from ADEN, specify your surface treatment requirements or describe your operating environment. For corrosive or hygienic applications, our standard practice includes pickling and passivation. Surface treatment is documented on the Certificate of Conformity that ships with every screen.