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 30 inner wall roughness, are prone to stress corrosion cracking and pitting corrosion. 31 Although the coating is usually deposited to protect steel pipe, it is particularly 32 challenging to deposit coatings on the inner walls of metal pipes with high aspect ratios 33 and complex shapes. In this work, nickel-phosphorus (Ni-P) coatings were successfully 34 applied on the inner walls of 304 stainless steel pipes with a high aspect ratio of 25:1 35 by using electroless plating within a circulated plating solution. The impact of the 36 plating solution flow rate on the microstructure uniformity and microhardness of the 37 deposited coatings was investigated. The coatings produced at a very low flow rate of 38 (0.04 mm/s) or very high flow rate of (> 66 mm/s) showed porous microstructure and 39 reduced hardness. On the contrast, the coating produced at a moderate flow rate of 40 40 mm/s exhibited dense microstructure with minimal microporosity (1.22%), the highly 41 uniform thickness along the axis of pipe, and the highest microhardness (660.2 HV). 42 After the Ni-P coatings prepared at the optimal flow rate were exposed under corrosive 43 hydrochloric acid solution at room temperature for 72 h, the weight loss of coated steel 44 pipes was only 51.8% of that from uncoated counterpart. The coated steel has a higher 45 corrosion potential (-110.98 mV) and lower corrosion current density (8.75 × 10⁻⁶ 46 A/cm²) compared to the uncoated steel. This indicates the successful processing and 47 significant improvement in corrosion resistance of the deposited coatings.