Inconel 690 vs 600: Nuclear & PWR Steam-Generator Tube Guide

Inconel 690 vs 600: Nuclear & PWR Steam-Generator Tube Guide

The Inconel 690 vs 600 question is, at its core, a story of nuclear steam-generator reliability. Inconel 600 (UNS N06600) served as the original steam-generator tubing alloy in pressurized-water reactors (PWRs), but decades of service revealed a fatal weakness: primary-water stress-corrosion cracking (PWSCC). Inconel 690 (UNS N06690) was developed specifically to eliminate that failure mode, using a much higher chromium content (27–31% vs 14–17%) and a tightly controlled low cobalt level. This guide explains the metallurgy, the data, and exactly where each alloy belongs in nuclear and high-purity steam service.

Both grades are nickel-chromium-iron solid-solution alloys with excellent high-temperature oxidation resistance, but their resistance to chloride and caustic cracking — and to activation in a neutron flux — is dramatically different. For a broader look at the 600 family versus its higher-strength cousin, see our Inconel 600 vs 625 comparison.

Why Inconel 690 Was Developed

In the 1960s and 70s, Inconel 600 was the standard tubing for PWR steam generators. It performed well against the corrosive secondary-side environment — until plants began discovering intergranular and transgranular cracking driven by primary water at the tube sheet and at crevices. This primary-water stress-corrosion cracking (PWSCC) eventually forced utilities into expensive steam-generator replacements and sleeving programs. Metallurgists identified chromium level as the dominant factor: raising chromium sharply improved resistance to both primary-water and caustic cracking.

The answer was Inconel 690, introduced in the 1980s with chromium raised to 27–31% and carbon/manganese tightened to keep it ductile and weldable. 690 has since become the preferred — and in many new-build designs, the mandatory — tubing and internals alloy for PWR steam generators, effectively retiring 600 from SCC-critical nuclear duties.

The underlying metallurgy is the stability of the passive chromium-rich oxide. At roughly 14–17% chromium, the protective film on 600 can break down locally under the combined chemical and mechanical driving forces of primary water, allowing cracks to nucleate and grow along grain boundaries. Pushing chromium to 27–31% thickens and stabilizes that film, raises the repassivation rate, and — combined with the low carbon and controlled grain structure — suppresses both intergranular and transgranular cracking. Extensive autoclave testing confirmed orders-of-magnitude lower crack growth rates for 690, which is what gave regulators and utilities the confidence to qualify it as the replacement material of record.

The scale of the original problem explains the industry’s decisive switch. By the 1990s, primary-water cracking had been found in a large fraction of operating PWR steam generators, and utilities faced multi-billion-dollar replacement programs decades ahead of schedule. Sleeving individual tubes bought time but added flow restriction and inspection burden. The consensus engineering response — codified in EPRI guidance and adopted across U.S., European and Asian fleets — was to retube or replace with 690 (typically a thermally treated variant) and to requalify all new SCC-critical internals on the higher-chromium alloy. Today, 690 is the default qualified material for both original equipment and replacement steam generators worldwide.

Inconel 690 (UNS N06690) Chemistry

The defining feature of 690 is its high chromium band combined with a deliberately low cobalt cap. Cobalt is limited to 0.10% maximum because, under neutron flux, cobalt-59 transmutes to cobalt-60, a hard gamma emitter that dominates personnel radiation dose during outage maintenance. Keeping Co low reduces activation of corrosion products circulated in the primary coolant.

Inconel 600 (UNS N06600) Chemistry

Inconel 600 is higher in nickel (72% min) and much lower in chromium (14–17%) than 690. That chromium shortfall is precisely why 600 is vulnerable to PWSCC; it is also why 600 remains valuable in non-SCC duties where its higher nickel content pays off — such as resistance to certain high-temperature oxidizing and nitriding atmospheres in heat-treat and furnace work.

In thermal-processing equipment, 600’s high nickel gives it an edge that 690 does not always need: excellent resistance to carburizing, nitriding and oxidizing atmospheres at temperatures up to roughly 1150–1200 °C, plus good strength for furnace trays, muffles, retorts and radiant-tube supports. Its relatively high nickel also makes it resistant to certain molten-salt and caustic environments encountered in chemical and electrochemical processing. For these non-nuclear, SCC-irrelevant duties, 600 is both qualified and economical, and there is no penalty in choosing it over 690. The rule of thumb is simple: if the failure mode is corrosion under sustained tensile stress in high-temperature water, specify 690; if it is high-temperature gas or salt exposure without that stress-corrosion mechanism, 600 remains a sound, lower-cost choice.

Mechanical & Density Comparison

Both alloys are annealed, solid-solution materials with similar yield but different densities owing to their chromium/iron balance. 690’s slightly lower density (8.19 vs 8.47 g/cm³) is a minor weight advantage in large tubing bundles, but the real differentiator is SCC behavior, covered next.

Thermally, both alloys share the low thermal conductivity and moderate coefficient of expansion typical of nickel-chromium materials, which is relevant to steam-generator design: the tubing must accommodate differential expansion between the tube and the tube sheet and between primary and secondary sides without inducing harmful stress. Their high chromium also improves resistance to oxidation and to the caustic and phosphate chemistries historically used on the secondary side. For mechanical design, the higher yield of 690 (about 45 ksi versus 35 ksi for 600) provides additional margin against denting, fretting and flow-induced vibration — all recognized degradation mechanisms in operating steam generators.

Stress-Corrosion Cracking: The Decisive Difference

Primary-water stress-corrosion cracking initiates when three conditions coincide: a susceptible microstructure, sustained tensile stress, and an aggressive high-temperature water chemistry (high-purity but hydrogenated primary coolant at ~290–325 °C). Inconel 600, especially in certain heat-treat conditions (sensitized or with coarse grain boundaries), proved susceptible, and cracking propagated over plant lifetimes. Inconel 690’s high chromium shifts the alloy into a regime where the passive film is far more stable and crack initiation is suppressed by orders of magnitude.

Equally important is secondary-side cracking (outer-diameter SCC from concentrated caustic or chloride at tube supports). Here too, 690’s chromium-rich film resists the combined crevice/chemical attack that defeated many 600 tubes. The net result: new PWR steam generators are almost universally tubed in 690 (or its TT — thermally treated — variant) to secure a 40–60 year design life.

The “TT” designation matters. Thermally treated 690 is given a controlled high-temperature heat treatment (typically around 700–715 °C for a defined time) after mill annealing, which optimizes grain-boundary chromium enrichment and further improves resistance to both primary-water and caustic cracking. Most modern nuclear specifications call for TT-690 specifically, because the combination of high chromium and the thermal-treatment microstructure delivers the highest verified immunity. This is a key procurement detail: ordering “690” without specifying TT may not meet the qualified condition demanded by the plant’s design basis.

Low Cobalt & Activated Corrosion Products

Beyond SCC, 690’s cobalt cap (≤0.10%) addresses a second nuclear-specific hazard. Cobalt in structural alloys wears and corrodes into the primary coolant, where neutron activation converts Co-59 to Co-60. Co-60 is a principal source of gamma radiation field buildup on primary piping and components, raising outage radiation doses for maintenance crews. By holding cobalt low, 690 minimizes the inventory of activatable material and helps plants meet ALARA (as-low-as-reasonably-achievable) dose targets. For this reason, many PWR specifications also limit cobalt in weld filler and neighboring alloys.

The dose-rate benefit is not only about the alloy itself. Activated cobalt-60 deposits on piping and components throughout the primary loop, so controlling the cobalt source term — from 690 tubing, from low-cobalt weld consumables, and from cobalt-free hardfacing where feasible — compounds the radiation-field reduction. Plants that converted to low-cobalt materials have measured meaningful decreases in outage radiation fields, shortening the time workers spend in contaminated areas and reducing collective dose. In an era where outage scheduling and dose budgets are tightly regulated, this secondary advantage of 690 is nearly as valued as its cracking immunity.

Where Each Alloy Belongs

The application split below reflects modern qualification practice. 690 owns the SCC-critical, activation-sensitive primary and secondary sides of PWR steam generators; 600 remains in legacy plants and in non-nuclear high-temperature roles where its chemistry is advantageous and SCC is not the limiting mechanism.

ASTM / ASME Standards & Codes

Both alloys are covered by parallel ASTM and ASME specifications, and nuclear use is further governed by ASME Section III (N-stamp) and, in France/Japan, RCC-M. Tubing for steam generators is most often supplied to ASTM B163 (seamless condenser and heat-exchanger tube) with nuclear-grade chemistry and melting controls.

Nuclear-grade supply carries additional controls beyond chemistry. Material is typically produced by electric-arc or vacuum-induction melting with stringent inclusion and trace-element limits, and finished tubing is subjected to eddy-current or ultrasonic examination, hydraulic test, and grain-size verification. Forged and plate products destined for ASME Section III construction require the “N” stamp and full material traceability, and weld filler must also meet low-cobalt and low-phosphorus requirements where specified. Procurement documents should therefore reference not only the ASTM form (B163/B168/B564) but also the governing code (ASME III or RCC-M) and any utility-specific specification, since “commercial-grade” 690 that meets the chemistry alone may not satisfy the qualified nuclear condition.

Selection Decision Framework

Use this sequence to choose between 690 and 600 for a nuclear or high-purity steam component:

1. Is the part SCC-critical? If it sees primary/secondary PWR water under sustained stress → specify 690.
2. Is activation control required? If the component is in or near the primary circuit → require 690’s Co ≤ 0.10%.
3. Is it a legacy 600 installation? Retain 600 only where already qualified and under inspection/sleeving programs.
4. Is the duty non-nuclear, high-temperature oxidation? 600 is acceptable for furnace/heat-treat hardware.
5. Select the form standard — B163 for tubing, B168 for plate, B564 for forgings — and the nuclear code (ASME III / RCC-M).
6. Require melting & chemistry certification confirming Cr band and Co cap before acceptance.

Frequently Asked Questions