The color of a titanium weld tells you exactly how much gas contamination occurred during welding. Silver means the purge worked perfectly. Straw means slight contamination that’s usually acceptable. Blue means the purge failed, and the weld is likely compromised. Purple, dark blue, or gray means you’re grinding it out and starting over. There’s no heat treatment, no extra weld pass, and no chemical cleaning that reverses oxygen and nitrogen absorption in titanium.
This color-to-contamination relationship is unique to titanium and makes visual inspection an immediately useful quality tool. But color inspection has limits: it only tells you about the surface you can see. Root-side contamination requires separate inspection of the back side of the joint.
How the Color Forms
When titanium is exposed to air above 500F, it absorbs oxygen and nitrogen into its surface. The absorbed gases form a thin oxide film (primarily TiO2) on the surface. The thickness of this film determines the visible color through thin-film interference, the same optical effect that makes soap bubbles and oil slicks show rainbow colors.
Thinner oxide films (less contamination, better shielding) produce lighter colors. Thicker films (more contamination, longer exposure to air at high temperature) produce darker colors. The color progression follows a consistent sequence:
| Color | Oxide Thickness (approx) | Oxygen in Surface (approx ppm) | What It Means |
|---|---|---|---|
| Bright silver | <10 nm | <50 | Excellent shielding; no measurable contamination |
| Light straw / light gold | 10-25 nm | 50-150 | Very slight contamination; generally acceptable |
| Dark straw / bronze | 25-40 nm | 150-300 | Marginal; acceptable for some commercial work |
| Light blue / violet | 40-60 nm | 300-600 | Contaminated; rejected for aerospace, marginal for commercial |
| Dark blue | 60-80 nm | 600-1000 | Heavily contaminated; rejected for all applications |
| Green-gray | 80-100 nm | 1000-2000 | Severe contamination; grind out completely |
| Gray / white (chalky) | >100 nm | >2000 | Extreme contamination; base metal may be affected |
The oxygen values in the table are approximate surface concentrations. The actual bulk oxygen content of the weld metal is lower because contamination concentrates at the surface. But surface embrittlement is where fatigue cracks initiate, so the surface condition matters more than the bulk composition for most applications.
Acceptance Criteria by Application
Different standards and applications have different color acceptance limits:
| Application / Standard | Acceptable Colors | Marginal (Review Required) | Rejected |
|---|---|---|---|
| Aerospace (AWS D17.1) | Silver, light straw | Dark straw (case-by-case) | Blue, purple, gray, any dark color |
| Chemical process (ASME B31.3) | Silver, straw, light blue* | Dark blue | Gray, white, powdery |
| Medical implants | Silver only | Light straw | Any color beyond light straw |
| Marine / general industrial | Silver, straw, light blue | Dark blue | Gray, white |
| Exhaust systems / non-critical | Silver through blue | Purple | Gray, white, flaking |
*Chemical process acceptance of light blue depends on the specific owner specification and service environment. Some specs reject anything beyond straw.
The critical point: even when a colored weld “passes” visual inspection for a given application, it’s always less than perfect. Silver is the target. Any color beyond silver means your purge procedure has room for improvement.
Color Inspection Procedure
When to Inspect
Inspect weld color immediately after each pass cools to below 200F. Don’t wait until the joint is complete because contamination on early passes gets buried by subsequent passes and becomes invisible.
Inspect both sides of the joint:
- Face side (torch side): Protected by primary gas cup and trailing shield. Color here tells you about cup coverage and trailing shield effectiveness.
- Root side: Protected by back purge. Color here tells you about purge gas quality and coverage. Root contamination is the most commonly missed defect because the root is harder to access for inspection.
Lighting Conditions
Inspect under bright, direct white light. Fluorescent shop lights and natural daylight both work. Avoid colored lights, dim lighting, or viewing through tinted welding lenses (except for comparison purposes). Some inspectors use a flashlight held at a shallow angle to highlight color variations.
Wet surfaces can mask or alter perceived color. Let the weld cool and dry completely before making acceptance decisions.
Documentation
For code work, document weld color with photographs taken under consistent lighting. Include a reference card (gray card or color standard) in the photo frame for calibration. Record the color observation for each pass, each side of the joint, at each clock position on pipe.
What Causes Each Type of Discoloration
Understanding the cause helps you fix the purge problem before welding the next joint.
Uniform Straw or Gold Along the Weld
Cause: Trailing shield gas flow slightly low, or the shield doesn’t extend far enough behind the torch. The weld is getting air exposure after it passes out of the trailing shield coverage but while it’s still above 500F.
Fix: Increase trailing shield flow by 5-10 CFH. If the shield is at maximum flow, extend its length by 2 inches or slow your travel speed so the weld cools more before leaving shield coverage.
Blue on the Face Side, Silver on the Root
Cause: Poor trailing shield coverage. The primary cup is doing its job (root is clean), but the trailing shield isn’t protecting the cooling bead.
Fix: Check trailing shield positioning. Verify it’s centered over the weld line and close enough to the surface (1/8 to 1/4 inch gap). Check for clogged diffuser holes. Increase flow rate.
Silver on the Face, Blue or Gray on the Root
Cause: Back purge failure. Either the oxygen level was too high when welding started, the purge dams leaked, or the purge flow was interrupted during welding.
Fix: Verify purge dam integrity. Re-check oxygen level with the analyzer. Inspect argon supply connections for leaks. Make sure the purge gas doesn’t get displaced when the root pass creates a draft through the pipe.
Spot Discoloration (Random Blue or Brown Spots)
Cause: Localized contamination source. Common culprits: fingerprints on the joint surface, oil droplet on the filler rod, a spatter ball from earlier work that off-gasses during welding, or a tiny leak in a gas hose fitting.
Fix: Review surface cleaning procedure. Wipe filler rod with acetone. Check all gas line connections. Clean the joint area more aggressively before the next weld.
Color Only at the Start of the Weld
Cause: Insufficient pre-flow time. The gas system wasn’t purged when the arc started, so the first inch of weld got contaminated before clean gas arrived.
Fix: Increase pre-flow time to 5-10 seconds. On trailing shields with separate gas supplies, open the trailing shield gas 5-10 seconds before striking the arc.
Color Only at the End of the Weld
Cause: Post-flow too short. The gas shut off before the end of the weld cooled below 500F.
Fix: Increase post-flow to 20-30 seconds minimum. On thicker material, 45-60 seconds may be needed. The trailing shield should continue flowing until the last inch of weld has cooled.
Effects of Contamination on Mechanical Properties
Weld color isn’t just cosmetic. The oxygen and nitrogen that create the color change also degrade mechanical properties:
| Property | Silver (Clean) | Straw | Blue | Gray |
|---|---|---|---|---|
| Hardness increase (HV) | 0 | +10-20 | +50-100 | +150-300 |
| Ductility (bend test) | Pass | Usually pass | Often fail | Always fail |
| Impact toughness | Full | 90-95% | 50-70% | <30% |
| Fatigue life | Full | 85-95% | 40-60% | <25% |
| Corrosion resistance | Full | Slightly reduced | Reduced | Severely reduced |
The hardness increase from oxygen absorption is the most measurable effect. A micro-hardness traverse across a contaminated weld shows a clear spike at the discolored surface compared to the clean interior. This hardened layer is where fatigue cracks initiate.
Repair of Contaminated Welds
There is no way to “fix” a contaminated titanium weld without removing it. The oxygen and nitrogen atoms are dissolved in the titanium lattice. They don’t come out with heating, pickling, or any other post-weld treatment.
Repair procedure:
- Mark the contaminated area based on color boundaries.
- Grind with a carbide burr or flap disc. Remove all discolored metal plus 1/16 inch of additional material beyond the visible color change (contamination extends slightly past the visible boundary).
- Inspect the ground surface. It should be bright silver titanium.
- Clean with acetone and lint-free wipe.
- Identify and fix the purge deficiency that caused the contamination.
- Reweld with full purge protection verified by oxygen analyzer.
- Re-inspect the new weld for color.
On critical applications, dye penetrant testing after grinding confirms complete removal of any contamination-related micro-cracking.
Limitations of Color Inspection
Color inspection is a powerful screening tool, but it has real limitations that inspectors and welders need to understand:
Color only reveals the surface. A weld can look silver on the outside while contamination exists at the root, at the fusion line, or between passes. Inspect both sides of the joint, and don’t rely on face-side color alone to approve a multi-pass weld.
Thin-film colors vary with viewing angle. The same oxide film can look straw at one angle and light blue at another. Inspect under consistent lighting conditions and try to view the surface at a consistent angle (roughly perpendicular).
Alloy composition affects the color scale. CP titanium and Ti-6Al-4V (Grade 5) produce slightly different colors at the same contamination level because the oxide film growth rate differs. The color chart is most accurate for CP grades. On alloyed titanium, use the color chart as a guide but confirm with hardness testing or other methods when the color falls in a borderline range.
Weld color doesn’t measure bulk composition. The color reflects surface oxygen content, not the bulk composition of the weld deposit. A weld that’s blue on the surface may still meet bulk chemistry requirements for oxygen content per ASTM B265, while a straw-colored weld might fail. For code work, color inspection supplements but doesn’t replace chemical analysis and mechanical testing of procedure qualification coupons.
For the full purge system setup including trailing shields, back purge dams, and equipment selection, see the titanium purge procedures guide. For welding parameters on CP grades, see welding commercially pure titanium.
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