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ASME STP-NU-068-2014 — Corrosion of A193 Grade B7 Bolt Material in BWR Sodium Pentaborate Solutions
📌 ASME STP-NU-068-2014 is a technical research report sponsored by the ASME Nuclear Codes & Standards and ASME Standards Technology, LLC (ASME ST-LLC), prepared by NWT Corporation at the request of the Electric Power Research Institute (EPRI). The report systematically evaluates the corrosion behavior of ASTM A193 Grade B7 — a quenched-and-tempered chromium-molybdenum bolting steel widely used in stainless steel valve flanges — when exposed to BWR (Boiling Water Reactor) sodium pentaborate shutdown solutions, providing an important engineering basis for judging the suitability of B7 bolting in BWR borated water systems and for planning in-service inspection programs.
💡 The background question is highly meaningful in practice: PWR borated water is well known to be corrosive, while BWR sodium pentaborate solution is alkaline. Whether BWR borated water is significantly less corrosive than PWR boric acid directly determines whether common high-strength bolts such as A193 B7 can continue to be used — this study was undertaken to answer that question.
1. Background and Scope 📌
During shutdown and refueling outages, BWRs inject boron into the reactor coolant to maintain sufficient shutdown margin. Sodium pentaborate is a commonly used boron source whose aqueous solutions are alkaline, in sharp contrast to the acidic boric acid (H3BO3) used in PWRs. The flanges of stainless steel valves and piping in BWR borated water systems are fastened with bolts that may be exposed to borated solution and humid atmospheres; the corrosion of that bolting had been questioned, motivating quantitative evaluation.
Specific items studied in the report:
- Bolt material: ASTM A193 Grade B7 (Cr-Mo alloy steel, roughly 0.38–0.49% C, 0.75–1.20% Cr, 0.15–0.25% Mo, minimum tensile strength about 125 ksi);
- Environment: approximately 12% sodium pentaborate solution at two temperature regimes — about 95°F (35°C) and 185–203°F (85–95°C) — representing normal shutdown hold conditions and heated borated-water conditions;
- Exposure forms: bare coupons, full-size bolts, and bolted flange assemblies.
⚠️ It must be emphasized that A193 B7 is a high-strength quenched-and-tempered steel: the higher the strength, the greater the potential sensitivity to environmental embrittlement and cracking. Accordingly, evaluation of B7 bolts cannot rely on uniform corrosion rate alone — crevice corrosion, pitting, and stress corrosion cracking must also be considered.
2. Test Methodology and Technical Requirements 🔬
The report follows a dual-path approach: coupon tests measure the intrinsic corrosion rate and weight-loss behavior of the material, while flange tests reproduce the crevices, contact, and preload conditions of real bolted joints. Coupon materials include C1010 mild steel and C4140, C4110, and C4340 steels typical of fasteners, together with actual A193 Grade B7 bolts exposed both as individual bolts and within flange assemblies.
| Test Type | Specimens / Assembly | Solution | Temperature | Primary Measurements |
|---|---|---|---|---|
| Low-temperature coupon tests | C1010, C4140 bare coupons | 12% sodium pentaborate | ~95°F (35°C) | Visual observation, weight loss, loss per unit area |
| High-temperature coupon tests | C1010, C4110, C4340, C4140 coupons | 12% sodium pentaborate | ~185°F (85°C) | Weight loss, corrosion morphology, two-month comparison |
| Low-temperature bolt exposure | A193 Grade B7 bolts | 12% sodium pentaborate | ~95°F (35°C) | Weight loss and surface condition after two months |
| Low-temperature flange tests | Stainless steel flanged bolt assemblies | Pentaborate injection | ~99°F (37°C) | Bolt weight change, deposits, crevice corrosion |
| High-temperature flange tests | Flange #4 bolt assembly | Pentaborate injection | 200–203°F (93–95°C) | Salt deposits, bolt shank diameter measurements, post-cleaning condition |
💡 The key to the test design is the two-temperature structure: about 95°F represents long-term hold after shutdown borating, while 185–203°F represents heated borated-water circulation or early shutdown conditions. The temperature dependence of the corrosion rate allows the accumulated corrosion to be estimated for different shutdown strategies.
3. Principal Results and Corrosion Behavior 📊
Over an exposure period of about 60 days, the report systematically recorded weight changes of coupons and bolts in the 12% sodium pentaborate solution. The tested steels generally showed low uniform corrosion rates; weight loss grew approximately linearly with time and increased with temperature. In the flange tests, visible salt deposits (borate precipitates) formed on the bolts; beneath the deposits a crevice-type environment existed, but after removal of the deposits and final cleaning the bolt shanks remained essentially intact. Diameter measurements of the bolt shanks confirmed that the corrosion reduction was within an acceptable engineering range.
Comparison of bolt weight changes between the low-temperature (about 95°F) and high-temperature (about 200°F) flange tests confirms that elevated temperature significantly accelerates corrosion, consistent with the Arrhenius behavior of electrochemical corrosion. The report provides tables of bolt weight change after the two-month exposure at 95°F and extensive photographic documentation (Appendices B and C), forming a complete evidence trail for quantitative evaluation.
| Assessment Dimension | Low Temperature (~95°F) | High Temperature (~185–203°F) | Engineering Conclusion |
|---|---|---|---|
| Uniform corrosion rate | Low; weight loss near-linear with time | Clearly higher; greater loss per unit area | Temperature dominates the corrosion rate |
| Surface condition | Slight discoloration, few deposits | Obvious salt deposits; base metal sound after cleaning | Deposits mask true corrosion; cleaning inspection required |
| Bolt shank reduction | Negligible | Within acceptable engineering range | Short-term exposure of B7 acceptable |
| Crevice / deposit effects | Minor | Localized environment under deposits | Long-term service must be confirmed by ISI |
4. Engineering Practice Essentials 🛠️
- 🔍 Material awareness: A193 B7 is the most common high-strength bolting steel in nuclear and petrochemical service. This report demonstrates controllable short-term corrosion in BWR pentaborate solution, but high-temperature and long-term duty still requires caution.
- 🔧 Crevice and deposit management: flange gaps, gasket edges, and thread roots concentrate corrosion; borate crystallization can create local crevices. Scheduled cleaning and visual inspection should eliminate these risks.
- 📊 Inspection interval: incorporate borated-water system bolting into the ASME Section XI in-service inspection program as controlled visual examinations, focusing on salt deposits, discoloration, and thread thinning.
- 📋 Material alternatives: where corrosion is a concern or inspection is difficult, evaluate austenitic stainless bolting (e.g., B8, B8M) or coated bolts, while re-verifying strength and high-temperature relaxation performance.
🚨 Engineering caution: when the borate solution evaporates or crystallizes on cooling, a strong crevice environment is created at the bolt/flange contact faces. If not cleaned periodically, local concentration under the deposit can lead to crevice corrosion or even cracking. Any decision to use B7 bolts in long-term borated-water immersion service must rest on periodic inspection and corrosion assessment.
5. Frequently Asked Questions ❓
❓ Q: Can A193 Grade B7 bolts be used in BWR sodium pentaborate systems?
A: This report shows that under short-term (about two months) exposure in 12% sodium pentaborate at 95–203°F, the weight loss and shank reduction of B7 bolts are within an acceptable engineering range. Long-term service, crevices, and elevated temperature combined still require confirmation through in-service inspection and further research.
A: This report shows that under short-term (about two months) exposure in 12% sodium pentaborate at 95–203°F, the weight loss and shank reduction of B7 bolts are within an acceptable engineering range. Long-term service, crevices, and elevated temperature combined still require confirmation through in-service inspection and further research.
❓ Q: Why do PWR boric acid and BWR sodium pentaborate differ in corrosivity?
A: PWR boric acid (H3BO3) is acidic and its acidic dissolution accelerates corrosion of carbon and alloy steels, whereas sodium pentaborate solutions are alkaline and promote passivation of steel surfaces, so corrosion rates are generally much lower — precisely the hypothesis this report verifies.
A: PWR boric acid (H3BO3) is acidic and its acidic dissolution accelerates corrosion of carbon and alloy steels, whereas sodium pentaborate solutions are alkaline and promote passivation of steel surfaces, so corrosion rates are generally much lower — precisely the hypothesis this report verifies.
❓ Q: Why are coupon results not directly equivalent to flange test results?
A: Coupons reflect intrinsic uniform corrosion rates; flange tests introduce preload, crevices, dissimilar-metal contact, and flow conditions that are closer to real service and are usually more conservative, making them more reliable for engineering judgments.
A: Coupons reflect intrinsic uniform corrosion rates; flange tests introduce preload, crevices, dissimilar-metal contact, and flow conditions that are closer to real service and are usually more conservative, making them more reliable for engineering judgments.
❓ Q: How large is the effect of temperature on bolt corrosion?
A: From about 95°F to about 200°F the bolt weight loss increases significantly, indicating a strong temperature dependence. Shutdown strategies and inspection plans should evaluate cumulative corrosion using the highest expected borated-water temperature.
A: From about 95°F to about 200°F the bolt weight loss increases significantly, indicating a strong temperature dependence. Shutdown strategies and inspection plans should evaluate cumulative corrosion using the highest expected borated-water temperature.
