ASTM B117 Salt Spray Test for…

What ASTM B117 actually measures

ASTM B117 is a comparative ranking test, not an absolute service-life predictor. The method subjects coated panels to a continuous salt fog under standardized conditions, measuring how long the coating resists visible corrosion relative to a control or acceptance threshold. It does not simulate real atmospheric exposure—B117 lacks UV, temperature cycling, and dry periods that dominate outdoor degradation. Its value lies in batch-to-batch consistency verification and pass/fail gatekeeping against specification requirements.

Chamber conditions: 35°C, 5% NaCl, pH 6.5-7.2

The test chamber operates at 35 ± 1.1°C with a salt solution of 5 ± 1% sodium chloride by weight. Solution pH must fall between 6.5 and 7.2 as measured at 25°C before atomization. The salt used must be NaCl with less than 0.1% sodium iodide and less than 0.3% total impurities—reagent-grade salt meets this easily, but some Indian labs use industrial-grade salt that introduces contamination-induced variability. The chamber maintains >95% relative humidity throughout the test duration.

Temperature control is critical. A chamber running at 37°C instead of 35°C accelerates corrosion disproportionately, producing artificially harsh results. Indian anodizers should request calibration certificates from their testing laboratories and verify thermocouple placement within the chamber zone containing test panels.

Spray collection rate and orientation

Per ASTM B117, fog collectors (typically 80 cm² horizontal funnels) must register a spray collection rate of 1.0 to 2.0 mL/hour. Rates below 1.0 mL/h indicate insufficient atomization; rates above 2.0 mL/h suggest excessive solution runoff that can wash protective films from panel surfaces. Test panels must be positioned at 15–30° from vertical to allow condensate drainage without pooling.

Panel placement within the chamber matters. Panels should not touch each other or chamber walls. The fog must reach all test surfaces uniformly—panels hidden behind others or placed in dead zones receive inconsistent exposure. Reputable labs rotate panel positions daily or use validated chamber designs with demonstrated uniformity.

Target hours by application

Different specifications mandate different anodize salt spray hours based on coating type, intended service environment, and safety criticality. Understanding these targets helps anodizers design process parameters appropriately rather than over-engineering every job.

Type II sealed: 336 hours

Standard sulphuric acid anodize (Type II per MIL-A-8625) with proper sealing should survive 336 hours of B117 exposure without rated failure. This applies to coating thicknesses of 18–25 µm with hot deionized water sealing or nickel acetate sealing. Well-executed anodize sealing methods on Type II coatings routinely achieve 400–500 hours before first visible corrosion, providing comfortable margin over specification minimums.

For thinner coatings (10–15 µm), the 336-hour target becomes marginal. Coatings below 15 µm thickness should be specified for shorter B117 durations or upgraded to 18+ µm if salt spray resistance is critical.

Hard anodize sealed: 500-1000 hours

Type III hard anodize coatings (≥50 µm thickness) per MIL-A-8625 are expected to survive 500–1000 hours depending on sealing method and customer specification. Unsealed hard anodize performs poorly in salt spray—the porous oxide wicks salt solution directly to the substrate. Sealed hard anodize at 50–75 µm thickness with hot water or dichromate sealing typically reaches 750–1000 hours before rated failure.

Some aerospace primes specify 1000+ hours for hard anodized hydraulic components. Achieving this requires coating thickness above 60 µm, excellent sealing quality (seal quality value <20 mg/dm² per ISO 2143), and contamination-free substrate preparation.

Architectural per AAMA 611 Class I

AAMA 611 Class I specifies minimum 18 µm coating thickness with sealed finish, requiring 336 hours B117 exposure without base metal corrosion, blistering, or coating failure. This specification governs most architectural anodizing in North America and is increasingly referenced for architectural anodizing in India on export projects.

Class II coatings (≥10 µm) face lower B117 requirements—typically 168 hours—but are restricted to interior applications or protected exterior locations. Indian anodizers bidding on international architectural work should confirm whether Class I or Class II applies before quoting.

Aerospace per MIL-A-8625

MIL-A-8625F Table II specifies corrosion resistance requirements by anodize type. Type I (chromic acid anodize) and Type IB (thin chromic) are tested per MIL-DTL-5541 (conversion coating test) rather than B117 for most applications. Type II and Type III coatings on 2000-series and 7000-series aluminium alloys require B117 testing when specified on the engineering drawing.

Typical aerospace Type II requirements range from 168 hours (Class 2 dyed coatings where corrosion protection is secondary to appearance) to 336 hours (Class 1 undyed coatings for corrosion-critical applications). Type III requirements commonly span 336–1000 hours depending on component criticality and operating environment.

Failure modes and how to read the panel

Interpreting B117 panels requires understanding what constitutes reportable failure versus acceptable test artifacts. Inspection occurs at intervals specified by the governing standard—commonly every 24 or 48 hours—with panels photographed and rated.

Pitting, white deposits, base-metal exposure

Primary failure modes for anodized aluminium in B117 include:

• Pitting corrosion: Localized attack penetrating the oxide layer, appearing as small dark spots that grow into crater-like defects. Pits indicate coating discontinuities or areas where sealing failed to close pore mouths adequately.
• White rust (aluminium hydroxide): Voluminous white deposits indicating active corrosion of the aluminium substrate. Unlike zinc white rust on galvanized steel, aluminium white rust signals complete coating failure at that location.
• Base metal exposure: Gray or metallic areas where the anodic coating has been completely undermined and lost. This is terminal failure—no further testing required at that location.
• Blistering or delamination: Coating lifting from substrate, typically starting at edges or mechanical damage points. Indicates inadequate pre-treatment or coating adhesion failure.

Not all white deposits indicate failure. Salt residue crystallization during inspection (when panels are removed from the chamber) can appear white but washes off without underlying damage. Trained inspectors distinguish corrosion products from salt residue by gentle rinsing before rating.

Edge/contact-mark exclusions

Standard practice excludes certain areas from rating:

• Panel edges within 6 mm of cut surfaces (saw-cut edges expose uncoated substrate)
• Rack contact points where handling fixtures touched the panel
• Identification markings (scribed numbers, paint dots)
• Known coating defects documented before test initiation

These exclusions recognize that production parts may have similar handling marks that don't affect functional coating performance. However, excessive edge creep (corrosion spreading >6 mm from edges) is reportable and suggests coating-thickness deficiency near edges—a common hard-to-reach area during anodizing.

Acceptance criteria

Acceptance criteria vary by specification:

1. AAMA 611: No more than 15 isolated pits none larger than 0.8 mm diameter in a 4 in² (2580 mm²) evaluation area; no base metal exposure; no blistering.
2. MIL-A-8625: Typically "no evidence of corrosion of basis metal" after specified hours, with exclusions for edges and contact points.
3. IS 1868: Indian standard references acceptance criteria similar to AAMA but with evaluation area specified in metric units; BIS-certified laboratories should reference the 2022 revision.

When rating panels, use consistent lighting (diffuse daylight or D65 equivalent), standardized viewing distance (300–500 mm), and documented rating methodology. Subjective "pass/fail" calls without quantified defect counts invite disputes.

Why a coating fails B117 — common root causes

B117 failures rarely result from single-cause problems. Root cause analysis should examine the entire process sequence, from substrate preparation through sealing and post-treatment handling.

Incomplete sealing

Sealing quality is the dominant factor separating coatings that pass B117 from those that fail. The anodic oxide is inherently porous—columnar pores 10–30 nm in diameter extend from surface to barrier layer. Unsealed pores act as capillary tubes, wicking salt solution directly to the aluminium substrate and initiating corrosion within hours.

Common sealing deficiencies include:

• Insufficient immersion time: Hot water sealing requires minimum 2–3 minutes per micron of coating thickness at 96–100°C. A 20 µm coating needs 40–60 minutes immersion—short-cycling to meet production targets produces inadequately sealed pores.
• Low sealing temperature: Hot water sealing below 96°C proceeds too slowly; nickel acetate sealing below 85°C produces incomplete hydration.
• Contaminated seal tank: Dissolved aluminium above 5 g/L, sulphate contamination above 100 mg/L, or organic contamination (oils from handling) all degrade seal quality.
• Inadequate rinsing before sealing: Acid dragout from the anodize tank neutralizes seal bath chemistry locally, producing poor seal quality at first-immersed surfaces.

Substrate contamination

Aluminium substrate contamination before anodizing produces coating defects that become B117 failure initiation sites:

• Rolling oils or drawing compounds: Residual lubricants not fully removed by degreasing produce localized anodize thinning or complete non-formation.
• Surface segregation: Intermetallic compounds (FeAl₃, MgSi, CuAl₂) in alloys like 2024 or 7075 anodize differently from the matrix, creating discontinuities.
• Water staining: Aluminium hydroxide formed during wet storage before anodizing disrupts uniform oxide growth.
• Fingerprints: Human skin oils leave chloride-containing deposits that cause pinpoint coating defects.

Thin coating in low-current-density areas

Anodize thickness varies with local current density. Recessed areas, internal corners, and surfaces shadowed by adjacent parts receive lower current density and develop thinner coatings. A nominal 20 µm coating on exposed surfaces may measure only 8–12 µm in recesses—below the threshold for reliable B117 performance.

Solutions include:

1. Racking parts to maximize exposure of critical surfaces to anodes
2. Using conforming anodes (auxiliary anodes positioned near recessed areas)
3. Extending anodize time to achieve minimum thickness everywhere, accepting heavier coating on exposed surfaces
4. Specifying minimum coating thickness at critical locations rather than nominal thickness overall

Practical QC: how often to run B117

B117 testing duration (336–1000 hours) makes it impractical for real-time process control. Effective QC programs balance B117's authoritative results against faster surrogate tests for routine monitoring.

Production sampling cadence

Typical sampling strategies for production anodizing lines include:

• Per-lot testing: One test panel per production lot when lot sizes are small or specifications demand 100% lot traceability (common in aerospace). Panels processed alongside production parts.
• Periodic testing: Weekly or bi-weekly B117 runs using dedicated test coupons anodized during normal production. Appropriate for architectural and industrial work with established process stability.
• Process-change testing: Mandatory B117 whenever sealing chemistry changes, anodize bath chemistry is rebuilt, or new alloy lots enter production.

Test coupons should be the same alloy as production parts—6063-T6 coupons don't validate process performance on 7075-T6 parts. Coupon surface condition (mill finish vs extruded vs machined) should match production parts.

Indian-context lab availability

NABL-accredited laboratories offering B117 testing exist in major industrial centers: Chennai, Bangalore, Pune, Mumbai, Ahmedabad, and Delhi-NCR. Typical B117 test costs range ₹2,500–4,500 per panel for 336-hour tests, with longer durations priced proportionally. Turnaround time equals test duration plus 2–3 days for evaluation and reporting.

For anodizers running frequent B117 tests, in-house salt spray chambers offer cost advantages. Entry-level B117-compliant chambers start around ₹3–4 lakh; fully automated chambers with continuous logging and NABL-calibration support run ₹8–15 lakh. Payback period depends on test volume—facilities running 10+ panels monthly typically justify in-house equipment within 18–24 months.

Cost vs alternative test methods

Faster screening tests can reduce B117 dependency for routine QC:

• Seal quality test (acid dissolution): Per ASTM B680 or ISO 2143, a 10-minute test measuring weight loss after phosphoric-chromic acid immersion. Detects incomplete sealing before B117 would reveal failures. Cost: ₹500–800 per test at commercial labs.
• Eddy-current thickness measurement: Per ASTM B244, non-destructive coating thickness verification in seconds. Does not assess seal quality but catches thickness-related B117 failures before they occur.
• Dye stain test: Quick visual check for seal quality—properly sealed coatings resist dye penetration. Qualitative only, useful for in-process checks.

An effective QC program combines daily thickness checks (eddy current), weekly seal quality tests (acid dissolution), and monthly B117 runs (or per-lot for critical applications). This layered approach catches problems early while maintaining B117 as the authoritative acceptance test.

FAQs

How many hours of ASTM B117 should a properly sealed Type II anodize survive?

AAMA 611 Class I specifies 336 hours minimum for architectural-grade Type II anodize at ≥18 µm thickness. Well-executed hot deionized water sealing or nickel acetate sealing on 18–25 µm Type II coatings typically survives 336–500 hours without rated failure. Coatings consistently failing before 336 hours indicate sealing deficiencies or coating-thickness shortfalls requiring process investigation.

Does B117 reflect real-world corrosion performance?

ASTM B117 is a relative-ranking screening test, not an absolute predictor of service life. The continuous salt fog at constant temperature differs fundamentally from outdoor exposure, which includes UV, temperature cycling, wet-dry transitions, and varying pollutant levels. B117 is useful for batch-to-batch comparison and specification compliance verification but should not substitute for natural-weathering data when predicting actual field performance.

What's the difference between ASTM B117 and cyclic salt spray?

ASTM B117 maintains continuous fog at constant 35°C conditions throughout the test duration. Cyclic salt spray tests (ASTM G85 Annex A2-A5, ISO 14993) alternate between salt spray, drying, humidity, and sometimes ambient cycles to better simulate outdoor exposure patterns. For anodized aluminium production QC, B117 remains the dominant method due to its established specification references and simpler equipment requirements, though cyclic tests provide more realistic corrosion morphology for research applications.