Plasma cladding Ni60 on Z2CN18-10 water guide bearings

Service context and material combination

This paper studies plasma cladding of Ni60 alloy on Z2CN18-10 nuclear grade stainless steel for water guide bearing surfaces. The base material is a low carbon austenitic stainless steel, and Ni60 is a nickel-chromium-boron-silicon hardfacing alloy. The combination is attractive because Ni60 provides hardness and wear resistance, while the stainless substrate provides structural strength and corrosion resistance in a water lubricated environment.

The study is useful because it does not stop at hardness. It also examines microstructural position, hardness distribution, and corrosion behavior in boric acid and simulated seawater. That is important for pump bearings, where wear resistance and electrochemical compatibility must be evaluated together. A hard overlay that is too cathodic or too active can create local galvanic cells with the substrate.

Microstructure and hardness

The Ni60 overlay contains gamma nickel, carbides, borides, and a gamma nickel plus boride eutectic. The paper notes that phase volume fractions differ across the bottom, middle, and top layers. The middle layer contains the most daisy-like eutectic structure. This is a meaningful observation because plasma cladding creates a steep thermal gradient. The bottom layer experiences more dilution and reheating, the middle layer cools under a more favorable solidification path, and the top layer may have different dilution and cooling behavior.

The hardness is about 500 HV, which is clearly higher than the stainless substrate. The paper links the daisy-like eutectic to hardness improvement. In practice, this means that process control must be aimed at producing a consistent middle-layer structure. If the cladding pass sequence, dilution, or cooling rate changes, the beneficial eutectic fraction may change. Hardness may remain acceptable, but wear life and crack resistance can shift.

The layer position also matters for machining and inspection. The top layer may be easier to inspect and may show different corrosion response than the bottom layer. If the final machined surface falls in a region with unfavorable phase distribution, the service performance may differ from the average reported value. This is a common issue in thick hardfacing.

Corrosion behavior and galvanic risk

The paper reports that Ni60 passivates more strongly in boric acid than in seawater. It also reports a small self-corrosion potential difference between Ni60 and Z2CN18-10 in boric acid, which reduces galvanic corrosion risk. In simulated seawater, the middle layer shows better corrosion resistance than the bottom and top layers, but the potential difference with the substrate is larger. That means galvanic corrosion becomes a concern.

This result is important for engineers designing pump bearings. In clean boric acid water, the overlay and substrate can behave in a relatively compatible way. In chloride containing water, the situation changes. Chloride attacks passive films, and local potential differences can accelerate attack at the interface or at exposed substrate near the overlay boundary. The middle layer may be more corrosion resistant, but if the bottom layer or interface is exposed, galvanic coupling can dominate the damage.

From a quality control standpoint, this suggests that corrosion testing should be performed on the actual finished surface condition, not only on a polished cross-section. Machining may expose a different layer than the original cladding top. Also, the exposed substrate near the cladding edge is a critical location. Fillet geometry, overlap, and interface cleanliness can influence galvanic attack.

Engineering practice and quality control

For water guide bearings, the paper supports a design review based on both wear and electrochemical compatibility. Ni60 is not simply a hard coating. It is a dissimilar metal layer on stainless steel. The weld procedure should control dilution so that the interface does not create a highly unfavorable galvanic couple. Post-cladding inspection should include visual and dimensional checks, and where required, corrosion coupons should be tested in the actual service water.

A practical process control point is to monitor cladding current, travel speed, and dilution. Too much dilution can change the bottom layer composition and increase interface risk. Too little fusion can create lack of bonding. The ideal middle-layer eutectic should not be pursued at the expense of bonding quality. A balanced procedure should specify maximum dilution, minimum hardness, and acceptable corrosion potential difference.

The study also suggests that seawater service requires more caution than boric acid service. If the bearing may see chloride water, consider an intermediate layer, improved passivation, or a corrosion compatible transition alloy. If the part is repaired in the field, the repair procedure should be qualified for the same corrosion environment.

Summary and study insight

The main lesson from this paper is that Ni60 plasma cladding on Z2CN18-10 stainless steel must be judged by its layered microstructure and its galvanic behavior, not by hardness alone. The overlay reaches about 500 HV, and the middle layer with daisy-like eutectic offers favorable properties. However, corrosion behavior changes with environment and layer position. In chloride containing water, a larger potential difference with the substrate can create galvanic corrosion risk. Engineers should therefore control dilution, inspect the finished machined surface, and qualify the repair procedure for the actual water chemistry.