You’ve just machined a batch of aluminum castings and you see defects. Small, round holes in some places. Dark, irregular spots in others. Maybe both.
These are two different problems: gas porosity and oxide inclusions. They look different, they form differently, and—most importantly—they require different fixes.
If you treat gas porosity like an inclusion problem, you’ll waste time on filtration when you should be degassing. If you treat oxide inclusions like a gas problem, you’ll degas until your ears ring and still see defects.
What You’re Actually Looking At
Gas Porosity (Hydrogen Porosity)
Gas porosity—often called hydrogen porosity or pinholes—is caused by dissolved hydrogen gas in the molten aluminum.
Aluminum has a high affinity for hydrogen. It absorbs hydrogen from moisture in the air, wet scrap, damp refractories, and even the humidity in your foundry. When the metal solidifies, hydrogen comes out of solution and forms small, round, smooth‑surfaced cavities evenly distributed throughout the casting.
What it looks like:
-
Round or elliptical holes
-
Smooth, shiny internal surfaces (sometimes oxidized, appearing light)
-
Usually less than 1 mm in diameter (pin‑hole porosity)
-
Distributed throughout the casting, not localized
Why it happens:
-
Moisture in the atmosphere reacts with aluminum to produce hydrogen
-
Wet or contaminated charge materials
-
Insufficient degassing
-
High humidity in the foundry environment
Oxide Inclusions (Bifilms, Dross, Oxide Films)
Oxide inclusions are non‑metallic particles—primarily aluminum oxide (Al₂O₃)—that become trapped in the casting.
The most damaging type is called a bifilm. When the oxidized surface of the melt gets folded back into itself during turbulent pouring or handling, it creates a double oxide film—a defect that acts like a crack inside the metal.
What it looks like:
-
Irregular, jagged shapes
-
Dark or grayish appearance
-
Often found near the top surface or in corners where flow is turbulent
-
Fracture surfaces appear grayish‑white or yellowish
Why it happens:
-
Turbulent pouring folds the oxide film back into the melt
-
Poor gating design causes splashing
-
Incomplete skimming leaves dross on the melt surface
-
Reoxidation during transfer
Side‑by‑Side Comparison
| Feature | Gas Porosity | Oxide Inclusions |
|---|---|---|
| Shape | Round or elliptical | Irregular, jagged |
| Surface | Smooth, shiny | Rough, dark or grayish |
| Size | Usually <1 mm | Variable—can be large films |
| Location | Throughout the casting | Top surfaces, turbulent areas |
| Primary cause | Dissolved hydrogen | Turbulence, poor melt handling |
| Reduced pressure test (RPT) | High porosity (many pores) | Low to moderate porosity |
| Effect on mechanicals | Reduces strength and ductility | Acts as crack initiation sites, severe reduction in properties |
The Connection Between the Two
Here’s the important part: oxide inclusions and gas porosity are not independent problems.
Oxide inclusions—especially bifilms—act as nucleation sites for hydrogen porosity. When hydrogen comes out of solution during solidification, it preferentially forms bubbles on the surface of existing oxide inclusions.
In other words, oxides give gas a place to start.
This means you can degas perfectly and still see porosity if your melt contains oxide inclusions. Conversely, you can filter out oxides and still see porosity if your hydrogen level is too high.
The two problems feed each other.
How to Diagnose Which Problem You Have
Test 1: Reduced Pressure Test (RPT)
The RPT is the standard test for hydrogen content. A sample of molten aluminum is poured into a small cup and allowed to solidify under reduced pressure (vacuum).
What it tells you:
-
High porosity in the RPT sample → Hydrogen is too high. Degas more.
-
Low porosity in the RPT sample, but defects in castings → Look for oxide inclusions.
Test 2: Visual Inspection (After Machining)
| If you see… | Likely… |
|---|---|
| Small, round, smooth holes | Gas porosity |
| Irregular, dark, or jagged spots | Oxide inclusions |
| Both | You have both—fix the oxides first |
Test 3: Location
| If defects are… | Likely… |
|---|---|
| Distributed throughout the casting | Gas porosity |
| Near the top surface or in corners | Oxide inclusions– |
| Near gates or turbulent areas | Oxide inclusions |
Test 4: Section and Fracture Surface
-
Gas porosity: Smooth, shiny internal surfaces
-
Oxide inclusions: Grayish or yellowish discoloration on fracture surfaces
How to Fix Each Problem
Fixing Gas Porosity
The only way to eliminate hydrogen porosity is to remove the hydrogen before pouring.
Degassing – Use a rotary degasser with argon or nitrogen. This is the most effective method. The rotor breaks the gas into fine bubbles that float through the melt, carrying hydrogen out with them.
Keep everything dry – Store charge materials in a dry place. Preheat ladles and tools. Control humidity in the foundry.
Monitor hydrogen levels – Use RPT regularly to verify that degassing is working.
Target – For most aluminum castings, aim for hydrogen below 0.12–0.15 ml/100g.
Fixing Oxide Inclusions
Oxide inclusions require prevention and filtration.
Reduce turbulence – Design gating systems to fill smoothly. Avoid splashing. Fill the sprue quickly and keep it full. Round sharp corners.
Skim thoroughly – Remove dross from the melt surface before pouring.
Use a ceramic foam filter – Place an alumina ceramic foam filter in the runner system, as close to the casting cavity as possible. The 3D structure captures oxide particles that other methods miss.
Proper placement – A filter placed in the wrong spot won’t help. Put it in the runner, after the sprue, where flow is stable.
The Fix That Addresses Both
Here’s the practical reality: most foundries have both problems to some degree. The most effective approach combines:
-
Degassing to remove hydrogen
-
Filtration to remove oxide inclusions
-
Good gating design to prevent new oxides from forming
When you remove oxide inclusions with a ceramic foam filter, you also remove the nucleation sites where hydrogen would otherwise form bubbles. The result is cleaner metal with fewer defects—even if you haven’t achieved perfect degassing.
Common Mistakes
Mistake #1: Degassing harder when the problem is oxides
If your RPT shows low hydrogen but you still have defects, degassing more won’t help. The problem is oxides. Add filtration.
Mistake #2: Adding a filter when the problem is hydrogen
A filter removes solid inclusions—not dissolved gas. If your RPT shows high porosity, you need to degas, not just filter.
Mistake #3: Assuming it’s only one problem
Remember: oxides nucleate gas. If you have oxides, you’ll also have more gas porosity than your hydrogen level would predict.– Fix both.
Conclusion
| Defect | Root Cause | Primary Fix |
|---|---|---|
| Gas porosity | Dissolved hydrogen | Degassing |
| Oxide inclusions | Turbulence, poor melt handling | Filtration + gating design |
Diagnose first:
-
RPT high → Degas
-
RPT low but defects persist → Filter
-
Both → Fix both
And remember: oxides and gas are connected. Removing oxides with a ceramic foam filter doesn’t just clean the metal—it removes the nucleation sites where gas would otherwise form.
At SF-Foundry, we manufacture alumina ceramic foam filters specifically designed to remove oxide inclusions from aluminum melts. Available in 10, 20, and 30 PPI, in a range of standard and custom sizes.
Contact SF-Foundry Technical Support:
Email: info@sf-foundry.com
Phone / WhatsApp: +86 18636913699
Website: www.sf-foundry.com