CLADDING TECHNOLOGY SHANXI CO., LTDPitting Potential and Critical Pitting Temperature Testing for Duplex and Super Austenitic Steels
Literature Overview
The evaluation of pitting resistance in high-performance stainless steels, particularly duplex stainless steels (such as UNS S31803 / S32750) and super austenitic grades (such as UNS S32750, Alloy C-276, and Alloy 254 SMO), represents a critical step in materials selection for aggressive chloride-containing environments. The two principal methodologies — electrochemical pitting potential measurement and FeCl₃ immersion testing — serve complementary roles in qualifying overlay materials and clad plate faces for service in marine, chemical processing, and nuclear applications. This study note synthesises the technical fundamentals, practical considerations, and engineering implications of these two approaches.
Core Technical Principles
Electrochemical Method
The electrochemical approach employs potentiostatic or potentiodynamic polarisation in a controlled chloride solution (typically 0.5–1.0 M NaCl at 25°C or elevated temperatures). The key parameters extracted from the polarisation curve are:
| Parameter | Definition | Typical Threshold |
|---|---|---|
| E_corr | Open circuit corrosion potential | Grade-dependent, typically −200 to +200 mV vs SCE |
| E_pit | Pitting potential (passive-to-active transition) | > +400 mV vs SCE for 2205 duplex; > +600 mV for super austenitic |
| E_rec | Repassivation potential | E_rec should be close to E_pit (low hysteresis) |
| I_pit | Pitting current density | < 10 µA/cm² indicates good resistance |
| ΔE (hysteresis) | E_pit − E_rec | Narrow hysteresis (< 50 mV) indicates stable passive film |
The potentiodynamic test involves scanning the potential at 0.5–2.0 mV/s in the anodic direction, observing the sudden current increase at E_pit. A reverse scan then determines E_rec. The critical pitting temperature (CPT) is determined by repeating the test at incrementally higher temperatures (typically in 5°C steps from 25°C to 80°C) until pitting initiates; the CPT is the highest temperature at which no pitting occurs within a defined exposure time (usually 15–30 minutes).
FeCl₃ Immersion Method
The FeCl₃ immersion test (ASTM G48, Method A) uses saturated ferric chloride solutions at specified temperatures and concentrations. The solution is prepared by dissolving FeCl₃·6H₂O in deionised water at 60°C, with the concentration adjusted by temperature to maintain saturation. The test specimen is immersed for a defined period (typically 24 hours), and pitting is evaluated by visual inspection and measurement of pit density and depth.
| Test Condition | Temperature | FeCl₃ Concentration | Typical Application |
|---|---|---|---|
| ASTM G48 Method A | 60°C | Saturated (~47%) | Standard comparison |
| ASTM G48 Method A | 80°C | Saturated (~40%) | More aggressive, differentiates grades |
| ASTM G48 Method A | 100°C | Saturated (~32%) | Extreme screening |
| Modified (lower Cl⁻) | 60°C | 5–20% | Mild conditions, closer to service |
Interpretation of Technical Points
Relationship Between PREN and Test Results
The Pitting Resistance Equivalent Number (PREN = %Cr + 3.3×%Mo + 16×%N) provides a useful screening criterion but does not directly correlate with measured pitting potentials. For example, Alloy 254 SMO (PREN ≈ 44) typically exhibits E_pit > +600 mV, while 2205 duplex (PREN ≈ 35) shows E_pit ≈ +400–500 mV. However, the actual performance in a specific environment depends on temperature, chloride concentration, pH, and flow conditions, which is why both electrochemical and immersion tests remain indispensable.
Influence of Heat-Affected Zone on Pitting Resistance
In weld overlay applications, the heat-affected zone (HAZ) of the overlay layer often exhibits reduced pitting resistance due to chromium carbide precipitation at grain boundaries. This phenomenon is particularly relevant for austenitic overlay layers (e.g., 308L, 309L, 316L) deposited on carbon steel substrates. The HAZ width and degree of sensitisation depend on the welding heat input, interpass temperature, and the amount of carbon in the filler metal. For duplex overlay systems, the HAZ is more complex because the phase fraction shifts from the optimal 50/50 α/γ balance toward either ferrite-rich or austenite-rich regions, potentially introducing σ-phase or χ-phase at elevated temperatures.
Engineering Practice Considerations
In practice, I have encountered several situations where the FeCl₃ immersion test results did not align with field performance:
- Temperature extrapolation: A material that passes the 60°C FeCl₃ test may still pit at 80°C in service if the chloride concentration is higher than assumed.
- Flow effects: Stagnant conditions in the test cell do not replicate flowing service conditions, where pit initiation may be accelerated by mechanical film disruption.
- Surface finish: The roughness of the overlay surface significantly influences pitting initiation. A rough surface (Ra > 3.2 µm) provides preferential sites for chloride accumulation and pit nucleation.
Key Questions and Reflections
- Why perform both electrochemical and immersion tests? The electrochemical method provides quantitative, reproducible data and allows rapid screening of multiple materials. The immersion test provides a more realistic assessment of pit morphology and density under prolonged exposure. Neither method alone is sufficient for critical applications.
- How to handle discrepant results? When electrochemical and immersion tests give conflicting rankings, the immersion test should generally be given more weight for engineering decisions, as it better simulates real exposure conditions. However, the electrochemical data should be used to identify the underlying mechanisms (e.g., whether the issue is related to passive film stability or metastable pitting).
- What is the role of CPT in design? The critical pitting temperature provides a practical design margin. For a service at 60°C, a material with a CPT of 75°C offers a 15°C margin, which is generally considered adequate. However, this margin must be adjusted for localised hot spots, dead legs, and areas of reduced flow.
Study Insights and Implications
The most significant insight from this study is that pitting resistance testing is not merely a pass/fail exercise but a tool for understanding material behaviour under specific service conditions. The engineer must carefully match the test conditions to the actual service environment, accounting for temperature, chloride concentration, pH, and flow. Furthermore, the results must be interpreted in the context of the manufacturing process — weld overlay introduces microstructural heterogeneity that can create localised zones of reduced pitting resistance, particularly at the interface between overlay layers and in the HAZ.
For bimetal pressure vessel fabrication, where overlay layers are deposited on carbon steel substrates, the pitting resistance of the overlay layer must be verified at the interface region, where dilution with the base metal can reduce the alloying content below the threshold required for adequate pitting resistance. This is particularly critical for duplex overlay systems, where the interface phase fraction may deviate significantly from the nominal composition.
In conclusion, the pitting potential and critical pitting temperature tests are essential tools for materials qualification, but their value is maximised only when the test conditions are carefully matched to service conditions and the results are interpreted with full awareness of the manufacturing history of the component. Engineers should always maintain a critical perspective, recognising that laboratory tests provide necessary but not sufficient evidence for service performance.