Corrosion Resistance of Nitrogen Alloyed Overlay Hardfacing Alloys

Literature Overview

The paper by Yang Ke, Yang Ke, and Bao Yefeng from Hohai University, published in Materials Engineering in 2015, investigates the corrosion resistance of nitrogen alloyed overlay hardfacing alloys based on martensitic stainless steel. The study introduces nitrogen into the martensitic stainless steel matrix and uses niobium, vanadium, and titanium as nitrogen-fixing elements to form nitrogen alloyed overlay hardfacing alloys. Electrochemical corrosion and chemical erosion experiments demonstrate that nitrogen alloying significantly improves the corrosion resistance of the hardfacing alloys by inhibiting chromium carbide precipitation and enhancing passivation film stability.

Nitrogen Alloying Mechanisms and Microstructural Effects

Nitrogen is a potent interstitial alloying element in stainless steels that provides multiple beneficial effects. The dissolution of nitrogen in the austenite or martensite matrix creates a strong solid solution strengthening effect, increasing the hardness and strength of the alloy. More importantly for corrosion resistance, nitrogen competes with carbon for chromium atoms, reducing the formation of chromium carbides at grain boundaries and within the matrix.

Chromium carbide precipitation is a primary mechanism for sensitization and intergranular corrosion in stainless steels. When chromium carbides such as Cr23C6 form at grain boundaries, they deplete the surrounding matrix of chromium, creating chromium-depleted zones that are susceptible to corrosion attack. By fixing nitrogen with niobium, vanadium, and titanium, the alloying elements form stable nitrides (NbN, VN, TiN) that prevent nitrogen from forming undesirable phases while simultaneously reducing the available carbon activity for chromium carbide formation.

Alloy Design and Composition

The microstructure of the nitrogen alloyed hardfacing alloy is characterized by a uniform and fine martensitic matrix with dispersed nitride particles. The nitrogen content shifts the martensite start temperature (Ms) to lower values, promoting the formation of retained austenite in some compositions. The retained austenite phase, stabilized by nitrogen, contributes to toughness and may enhance corrosion resistance by providing a more uniform electrochemical potential across the microstructure.

The nitride particles formed by Nb, V, and Ti are thermodynamically stable and do not dissolve during the welding thermal cycle or subsequent service exposure. These particles act as inert second phases that do not participate in corrosion reactions, while the surrounding matrix maintains a uniform chromium distribution. The fine and uniform distribution of these particles prevents the formation of galvanic couples that could accelerate localized corrosion.

Electrochemical Corrosion Performance

The electrochemical testing reveals that nitrogen alloying significantly improves the corrosion resistance of the hardfacing alloy. The self-corrosion potential shifts from -345 millivolts to -264 millivolts versus a reference electrode, indicating a more noble potential and enhanced thermodynamic stability. This positive shift in potential is attributed to the improved passivation film stability resulting from the inhibition of chromium carbide precipitation.

The passivation film on nitrogen alloyed stainless steels is more stable and uniform due to the higher chromium content available for film formation. Nitrogen itself also contributes to passivation film stability by promoting the formation of a more compact and adherent chromium oxide film. The presence of nitrogen in the film structure has been shown to reduce the film dissolution rate and increase the repassivation rate after film damage.

Electrochemical Parameters Comparison

The polarization curves show that the nitrogen alloyed alloy exhibits a wider passivation region and lower passivation current density, indicating a more stable and protective passive film. The breakdown potential, which indicates the onset of pitting corrosion, is shifted to more positive values, demonstrating enhanced resistance to localized corrosion initiation. The hysteresis between the anodic and cathodic branches of the polarization curve is smaller for the nitrogen alloyed alloy, indicating a more reversible passivation process.

Chemical Corrosion Testing in FeCl3 Solution

The chemical corrosion testing in iron(III) chloride solution provides a complementary assessment of pitting corrosion resistance. The FeCl3 solution is an aggressive pitting corrosion medium that simulates chloride-containing environments such as seawater, industrial atmospheres, and chemical processing conditions. The results show that the nitrogen alloyed hardfacing alloy develops smaller and more dispersed pitting corrosion sites compared to the conventional alloy.

The uniform and fine microstructure of the nitrogen alloyed alloy is directly responsible for the improved pitting resistance. The absence of chromium carbide precipitates at grain boundaries eliminates the preferential sites for pitting initiation. The dispersed nitride particles do not create galvanic couples with the matrix, and the fine grain structure provides a more uniform electrochemical surface. The result is a corrosion pattern characterized by numerous small, shallow pits rather than a few large, deep pits that can lead to rapid component failure.

Pitting Corrosion Morphology

The pitting corrosion mechanism in the conventional alloy involves preferential attack at chromium carbide-rich zones and grain boundaries, leading to deep and localized pits. In contrast, the nitrogen alloyed alloy exhibits uniform surface attack with shallow, dispersed pits that do not rapidly penetrate the material. This difference in corrosion morphology has significant implications for component design and service life prediction, as the nitrogen alloyed alloy provides more predictable and gradual material loss rather than sudden failure from deep pitting.

Engineering Implications and Application Potential

The nitrogen alloying approach offers a practical pathway to enhance the corrosion resistance of overlay hardfacing alloys without compromising their wear resistance. The nitrogen content can be controlled during the welding process through the selection of welding consumables containing nitrogen or through the use of nitrogen-containing shielding gases. The addition of Nb, V, and Ti as nitrogen-fixing elements is straightforward and does not require specialized equipment or processes.

The improved corrosion resistance of nitrogen alloyed hardfacing alloys makes them suitable for applications where both wear and corrosion resistance are required, such as pump impellers, valve seats, marine components, and chemical processing equipment. The combination of high hardness (55 to 65 HRC) and enhanced corrosion resistance provides a unique performance envelope that is difficult to achieve with conventional hardfacing alloys.

The research findings have broader implications for the design of overlay welding consumables. The principle of using nitrogen as a carbide inhibitor and passivation enhancer can be extended to other stainless steel and superalloy systems. The concept of nitrogen fixation with refractory metals such as Nb, V, and Ti provides a general strategy for controlling nitrogen-related phase formation in weld deposits.

The practical implementation of nitrogen alloyed hardfacing alloys requires careful control of the nitrogen content to avoid excessive retained austenite or nitrogen gas porosity in the weld deposit. The optimal nitrogen content is typically in the range of 0.1 to 0.5 percent, above which the risk of porosity and excessive retained austenite increases. Welding procedure qualification must account for the effects of nitrogen on weld pool fluidity, spatter, and post-weld residual stress.

This research demonstrates the power of microalloying in improving the performance of welding consumables. The addition of small amounts of nitrogen, combined with strategic nitrogen-fixing elements, produces significant improvements in corrosion resistance without requiring major changes to the base alloy composition or welding process. For materials engineers and welding metallurgists, this approach offers a cost-effective and practical solution to the challenge of combining wear and corrosion resistance in surface engineering applications.