Optimizing Electroless Ni–P Coating on the Inner Wall of 304 Stainless Steel Pipes: Control of Plating Solution Flow Rate for Enhanced Microstructural Uniformity and Corrosion Resistance

Cold-drawn stainless steel pipes, characterized by high residual stress and high inner wall roughness, are prone to stress corrosion cracking and pitting corrosion. Although the coating is usually deposited to protect steel pipe, it is particularly challenging to deposit coatings on the inner walls of metal pipes with high aspect ratios and complex shapes. In this work, nickel–phosphorus (Ni–P) coatings were successfully applied on the inner walls of 304 stainless steel pipes with a high aspect ratio of 25:1 using electroless plating within a circulated plating solution. The impact of the plating solution flow rate on the microstructure uniformity and microhardness of the deposited coatings was investigated. The coatings produced at a very low flow rate of (0.04 mm/s) or very high flow rate of (> 66 mm/s) showed porous microstructure and reduced hardness. On the contrast, the coating produced at a moderate flow rate of 40 mm/s exhibited dense microstructure with minimal microporosity (1.22 pct), the highly uniform thickness along the axis of pipe, and the highest microhardness (660.2 HV). After the Ni–P coatings prepared at the optimal flow rate were exposed under corrosive hydrochloric acid solution at room temperature for 72 hours, the weight loss of coated steel pipes was only 51.8 pct of that from uncoated counterpart. The coated steel has a higher corrosion potential (− 110.98 mV) and lower corrosion current density (8.75 × 10⁻⁶ A/cm²) compared to the uncoated steel. This indicates the successful processing and significant improvement in corrosion resistance of the deposited coatings.