Alloy desulfurization nozzle wear resistance testing is a specialized materials evaluation service for spray nozzles operating in wet flue gas desulfurization (FGD) systems, where abrasive limestone-gypsum slurries, aggressive chloride chemistry, and high-velocity droplet impingement combine to cause severe erosion and corrosion damage. By applying standardized procedures such as ASTM G65, ASTM G75 and ASTM G32, accredited third-party laboratories can quantify wear rates, verify material quality, and predict the service life of nickel-based, duplex and coated alloy nozzles. This article explains the scope of testing, typical sample types, key test items, standardized methods, acceptance criteria and common industrial applications.
Technical Overview
Alloy desulfurization nozzles are precision spray components installed in the absorber towers of wet FGD systems at coal-fired power plants, steel sintering lines, waste-to-energy facilities and marine exhaust gas scrubbers. In service, they continuously atomize limestone or lime slurries carrying a high concentration of suspended solids, typically 15 to 25 percent by weight of gypsum, limestone and fly ash particles, at line pressures of roughly 0.07 to 0.2 MPa. The resulting high-velocity flow exposes the swirl chamber, vanes and orifice to a demanding combination of degradation mechanisms.
The dominant damage modes are three-body sliding abrasion inside the internal flow passages, particle erosion at flow-turning surfaces and the orifice edge, cavitation and droplet impingement erosion, and a strongly synergistic erosion-corrosion interaction driven by acidic chloride-bearing liquor at pH values of roughly 4.5 to 5.8, where chloride concentrations in closed-loop systems can exceed 20,000 mg/L. Progressive orifice enlargement degrades the spray pattern, reduces tower coverage, disturbs gas-liquid contact and ultimately compromises sulfur dioxide removal efficiency and absorber reliability.
Wear resistance testing therefore serves three essential purposes: qualification of candidate alloys and coatings before plant installation, batch quality control during nozzle manufacturing, and remaining-life assessment of nozzles recovered from service. Testing is normally performed by accredited third-party laboratories against recognized ASTM, ISO, EN and GB standards, providing objective, comparable data for material selection and maintenance planning.
Testing Samples
Nickel-Based Alloy Nozzles
Solid-solution strengthened nickel-chromium-molybdenum alloys such as UNS N10276 and UNS N06625, supplied as castings, fabrications or machined components. These grades combine high slurry abrasion resistance with exceptional resistance to chloride-induced pitting and crevice corrosion, and are frequently specified for the most aggressive FGD liquors.
Duplex and Super Duplex Stainless Steel Nozzles
Grades such as UNS S32205 and UNS S32750 offer a favorable combination of strength, erosion resistance and corrosion resistance. Testing typically includes verification of the austenite-ferrite phase balance, which strongly influences both mechanical integrity and corrosion performance in acidic chloride media.
Super Austenitic Stainless Steel Nozzles
High-alloy austenitic grades such as UNS N08904 and the six-percent-molybdenum grade UNS S31254 are common mid-tier solutions, and their wear and corrosion behavior is benchmarked against both standard stainless steels and nickel alloys during material qualification programs.
Surface-Engineered and Coated Alloy Nozzles
Alloy substrates protected by high-velocity oxy-fuel sprayed tungsten carbide-cobalt or WC-CoCr cermet coatings, laser-cladded overlays, or diffusion-treated layers. These systems require additional evaluation of coating adhesion, porosity, hardness gradient and interface integrity.
Specimen Forms
- Finished or field-returned nozzles for dimensional inspection, surface roughness mapping and non-destructive evaluation.
- Sectioned pieces cut from nozzle bodies for microhardness profiling and metallographic examination.
- Machined flat coupons taken from the same heat, batch or coating run as the production nozzles, used as witness specimens in standardized wear and corrosion apparatus.
- Reference materials of known behavior, such as standard stainless steel coupons, tested alongside the samples to normalize and validate results.
Testing Items
Depending on the engineering specification, material class and failure mode under investigation, the following items are typically evaluated:
- Dry abrasive wear resistance, expressed as volume loss and normalized wear rate, per ASTM G65.
- Slurry abrasion response of the material and slurry abrasivity characterization per ASTM G75.
- Cavitation erosion resistance per ASTM G32 or ASTM G134.
- Liquid impingement and slurry-jet erosion resistance per ASTM G73 and jet-rig methods.
- Bulk hardness by Rockwell and Vickers methods, plus micro-Vickers or Knoop hardness depth profiles across coatings and heat-affected zones.
- Coating adhesion strength by tensile pull-off per ASTM C633, and coating porosity by image analysis.
- Microstructure, grain size, phase balance, carbide distribution and inclusion rating.
- Phase constitution by X-ray diffraction, including retained austenite content and binder condition in cermet coatings.
- General and localized corrosion resistance by immersion testing per ASTM G31 and by electrochemical methods including open-circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy.
- Salt spray corrosion resistance per ASTM B117, ISO 9227 or GB/T 10125 where coastal or marine service is anticipated.
- Erosion-corrosion synergism evaluation following the framework of ASTM G119.
- Surface roughness before and after exposure, dimensional verification of orifice diameter, spray angle and internal vane geometry, and spray pattern uniformity on a spray bench.
Testing Methods
Dry Sand/Rubber Wheel Abrasion (ASTM G65)
The specimen is pressed against a rotating rubber wheel under a defined normal force while graded silica sand is fed into the contact zone. After a set number of wheel revolutions, selected from procedures A through F according to material hardness and expected wear, the mass loss is converted to volume loss using the measured density. This provides a fast, repeatable ranking of alloys and coatings; hard carbide coatings typically record volume losses an order of magnitude lower than standard austenitic stainless steel.
Slurry Abrasion Response (ASTM G75)
Specimens reciprocate inside a tray containing a standardized sand-and-water slurry while being loaded against the tray bottom. Mass loss is measured after each two-hour cycle, normally for three cycles, and the mass-loss curve yields the slurry abrasion response number for the material and the Miller number for the slurry. This method is particularly relevant to FGD nozzle service because it reproduces three-body abrasion in a wet medium, and the slurry chemistry can be adapted to plant-specific limestone-gypsum liquor with controlled chloride content.
Cavitation Erosion (ASTM G32 and ASTM G134)
In the ultrasonic vibratory method of ASTM G32, a cylindrical specimen about 16 mm in diameter is attached to a horn oscillating at approximately 20 kHz while immersed in water or simulated process liquor, generating repetitive cavitation bubble collapse at the surface. Cumulative mass loss versus time and the mean depth of erosion rate characterize resistance to cavitation attack at the swirl chamber and orifice edges. ASTM G134 offers a submerged cavitating-jet alternative that produces comparable material rankings.
Liquid Impingement and Slurry-Jet Erosion (ASTM G73 and Jet Rigs)
Impingement rigs accelerate liquid droplets or slurry jets onto specimens at controlled velocities, commonly 20 to 150 m/s, and at selected impact angles such as 30 and 90 degrees. This reproduces droplet hammering and particle cutting at the nozzle exit. Results are reported as erosion depth per unit time or mass loss per unit mass of impinging solids, and the dependence on impact angle distinguishes ductile from brittle material response.
Hardness and Microhardness Testing
Bulk hardness is measured according to ASTM E18, ASTM E92, ISO 6507, ISO 6508, GB/T 230.1 or GB/T 4340.1 as appropriate. Microindentation traverses per ASTM E384 or ISO 14577 map hardness from the coating surface through the interface into the substrate, confirming effective layer thickness, coating quality and the absence of over-tempering or softening in heat-treated nozzle bodies.
Corrosion and Erosion-Corrosion Evaluation
Immersion tests per ASTM G31 in simulated FGD liquor, for example chloride levels from 1,000 to 30,000 mg/L at pH 4.5 to 5.8 and 50 to 70 degrees Celsius, establish uniform corrosion rates and susceptibility to pitting and crevice attack. Electrochemical polarization quantifies pitting potential and passive current density, while ASTM G119 provides a systematic framework for separating pure erosion, pure corrosion and their synergistic contribution by comparing slurry erosion tests with and without electrochemical monitoring. Salt spray exposure per ASTM B117, ISO 9227 or GB/T 10125 supplements the program for marine scrubber applications.
Metallographic, Structural and Dimensional Analysis
Sections prepared according to ASTM E3 and GB/T 13298 are examined for grain size per ASTM E112 or GB/T 6394, duplex phase balance, carbide morphology, coating porosity, inclusions, cracks and interface defects. X-ray diffraction verifies phase constitution, and quantitative image analysis reports porosity levels. Nozzle geometry is verified with optical comparators, coordinate measuring machines and spray benches, comparing orifice diameter, spray angle and liquid distribution uniformity against design drawings and pre-service baselines.
Reporting and Acceptance Criteria
A complete test report documents material certification, test conditions, equipment calibration status, mass and volume loss data, normalized wear rates, hardness maps, micrographs and corrosion results. Acceptance is typically defined by comparison with a qualified reference material or by contractual limits, for example a volume-loss ratio not exceeding an agreed multiplier of the reference, hardness within the specified band, coating adhesion above the stated minimum, correct phase balance and porosity, and no cracking, delamination or localized corrosion deeper than the permitted depth.
Applications
Wear resistance testing of alloy desulfurization nozzles supports engineering and procurement decisions across a wide range of industries:
- Wet limestone-gypsum FGD absorbers in coal-fired and biomass power plants, where thousands of spray nozzles determine sulfur dioxide removal efficiency and absorber reliability.
- Semi-dry and circulating fluidized bed desulfurization systems in steel sintering plants and iron ore pelletizing lines.
- Waste incineration and hazardous waste combustion flue gas scrubbing trains.
- Marine exhaust gas cleaning systems on ships, where hot acidic seawater imposes severe combined erosion and corrosion loading.
- Chemical, petrochemical and refinery gas scrubbing towers handling abrasive slurry or sludge streams.
- Supplier qualification, incoming inspection, failure analysis of worn or cracked nozzles, and remaining-life prediction for predictive maintenance programs.
FAQ
Which wear test best simulates FGD slurry service?
ASTM G75 slurry abrasion testing most closely reproduces three-body wet abrasion, and it can be run with plant-specific limestone-gypsum liquor chemistry. ASTM G65 is widely used for fast and repeatable material ranking, while ASTM G32, ASTM G134 and slurry-jet rigs address cavitation and high-velocity droplet erosion at the orifice. A combined program covering abrasion, erosion and corrosion synergy gives the most complete picture for critical installations.
What specimens are required and how large are they?
ASTM G65 uses rectangular coupons on the order of 25 mm wide and up to 100 mm long with several millimeters of thickness; ASTM G32 uses cylindrical buttons about 16 mm in diameter; and ASTM C633 adhesion specimens are 25.4 mm diameter cylinders. Finished nozzles can be accepted directly for dimensional, roughness and metallographic examination, with machined witness coupons from the same heat or coating batch used for the destructive wear and corrosion tests.
How long does a typical test program take?
Individual wear runs range from roughly thirty minutes for short ASTM G65 procedures to six or more hours for ASTM G75 or steady-state cavitation programs. Including specimen preparation, conditioning and reporting, a wear-only evaluation typically completes within five to ten working days, while programs combining immersion corrosion or salt spray exposure may require two to four weeks.
How are results judged as pass or fail?
Acceptance is defined by the purchaser's specification or the agreed test plan: volume or mass loss within a stated limit or within a specified ratio of a reference material, hardness inside the specified range, coating adhesion above the minimum value, correct phase balance and porosity, and absence of cracks, delamination or localized corrosion deeper than allowed. All results are reported together with the test conditions and, where applicable, measurement uncertainty so that they can be audited and compared between laboratories.
Can nozzles already in service be evaluated?
Yes. Field-returned nozzles are cleaned, weighed and dimensionally mapped against pre-service records to calculate cumulative wear and to extrapolate the orifice growth rate. Sectioned areas then reveal whether the dominant mechanism is abrasion, particle erosion, cavitation or corrosion-assisted material removal, and this evidence guides material upgrades, for example moving from standard stainless steel to super duplex, nickel alloys or carbide-coated designs.
