26/07/2026
π¬ THE SCIENCE BEHIND NHOU
EPISODE 18
Why Are Welded Seams the First Place to Rust?
If you inspect an old vehicle, you'll notice something interesting.
Rust rarely starts in the middle of a flat steel panel.
Instead, it almost always appears around:
β’ welded seams;
β’ spot welds;
β’ folded edges;
β’ overlapping metal panels.
This is no coincidence.
Why are welded seams so vulnerable?
A vehicle body isn't made from one large sheet of steel.
It's assembled from hundreds of individual parts that are welded together.
Where two steel panels overlap, a very narrow gap remains.
It may only be a fraction of a millimeter wideβbut that's enough.
Water and road salt are drawn into these tiny gaps by capillary action.
Once inside, they remain trapped much longer than on an exposed surface.
Moisture can't escape easily
Sunlight can't reach these cavities.
Air circulation is minimal.
After driving in rain or snow, the outer surface may dry within hours.
Inside the seam, moisture can remain for days.
That gives corrosion plenty of time to work.
Salt makes everything worse
Road salt doesn't simply disappear after the vehicle dries.
Water evaporates.
Salt crystals stay behind.
The next time moisture enters the seam, those crystals dissolve again, recreating an electrolyte that restarts electrochemical corrosion.
This cycle repeats throughout the vehicle's life.
Why factory protection isn't enough forever
Modern vehicles leave the factory with excellent corrosion protection.
But every vehicle flexes during normal driving.
The body twists slightly over bumps.
Doors close thousands of times.
Temperature changes make steel continuously expand and contract.
Over the years, microscopic cracks can develop in factory sealers and coatings around welded joints.
They are invisible.
But they are large enough for moisture to enter.
Corrosion begins where you cannot see it
At first, nothing appears unusual.
The paint still looks perfect.
Monthsβor even years laterβa tiny rust bubble appears.
What you see on the outside is often only a small part of the damage already developing inside the seam.
Simply put:
A welded seam is like two wet pages pressed tightly together.
The outside dries quickly.
The moisture trapped between them stays much longer.
The same thing happens between welded steel panels.
And wherever moisture stays, corrosion follows.
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