For stainless steel products, it’s easy to assume corrosion performance is “built in.” Our recent project proved the opposite: surface preparation choices can make or break salt spray results—even when the base material is 304 stainless steel.
This post shares a real case we worked through, what went wrong, what we changed, and the practical lessons we’ll carry into every future program.
Background: The Original Requirement Looked Simple
The customer’s initial request was straightforward:
- Sand blasted surface (matte/texture requirement)
- Passivation
- Later, after the texture was approved: 48-hour Salt Spray Test (SST) requirement
In real life, these “simple” requirements can conflict. Texture often pushes processes toward more aggressive blasting. But corrosion performance requires strict control of contamination and surface chemistry.
What We Were Trying to Solve
The customer strongly preferred a rougher, more defined blasted texture.
When we achieved that look, the next requirement arrived:
- Pass 48-hour SST consistently
At this point, we were balancing three competing targets:
- Appearance (rough texture)
- Corrosion performance (48h SST)
- Production stability (repeatable in volume, not just in samples)
What We Found (The Turning Point)
As we continued refining the appearance while protecting SST performance, we brought in an external perspective and reviewed the process details more deeply—especially blasting media selection.
That’s when we discovered the blasting shop had been using:
- Carbon steel grit for blasting 304 stainless steel
Why does this matter?
Because carbon steel grit can introduce ferrous contamination, including mechanically embedded iron particles. These particles may not always be visible, and they don’t always cause immediate failures—yet they can create long-term corrosion risk and inconsistency in salt spray results, especially in complex geometry and threaded areas.
“But the Samples Passed 48 Hours…”
This is where many teams stop.
In our case:
- The supplier’s internal SST passed
- A third-party lab SST also passed
- The parts had undergone electropolishing + passivation
So why were we still concerned?
Because one successful test does not automatically guarantee long-term production stability when a known risk mechanism exists. Salt spray is highly sensitive to “weakest points,” and embedded ferrous particles (or process variability in removing them) can show up later as sporadic failures.
Electropolishing vs. Passivation: What Each One Actually Does
One reason these issues become confusing is that people treat “EP” and “passivation” as interchangeable. They’re not.
Electropolishing (EP)
Electropolishing is an electrochemical removal of a thin surface layer. It can:
- Reduce surface free iron and machining smear
- Improve surface cleanliness and corrosion behavior
- Help lower corrosion risk—often dramatically
But EP cannot be assumed to eliminate every mechanically embedded ferrous particle introduced by carbon steel grit, especially in geometric shadow areas where current density is less uniform.
Passivation
Passivation is a chemical treatment intended to:
- Remove light surface iron contamination
- Promote formation of a stable passive film
Passivation is not a substitute for controlling upstream contamination sources. It works best when the surface is already clean and properly prepared.
The Real Root Cause
Looking back, the issue wasn’t “someone made a mistake.” It was a requirements and process-control gap:
- The surface texture requirement became the dominant target.
- The blasting media choice was not explicitly controlled or documented.
- The 48-hour SST requirement was added after the texture was finalized.
- We were optimizing “how it looks” and “how it tests” separately, rather than locking a process that guarantees both.
What We Changed (Our Corrective Strategy)
To make the process both stable and defensible for volume production, we rebuilt our approach around process boundaries, not just end results.
Here’s what we now treat as non-negotiable in stainless steel blasting programs:
- Blasting media must be explicitly specified
- Non-ferrous / non-metal media (e.g., glass bead) or stainless media when applicable
- No “default media” assumptions
- Cleaning is process control, not an afterthought
- Post-blast cleaning to remove residual media and dust
- Post-machining cleaning/degreasing so passivation can work properly
- EP is a tool—use it intentionally
- EP can significantly improve corrosion resistance
- But it should not be used as a “blanket fix” to justify high-risk upstream steps
- SST should be managed at the lot level
- For destructive/time-consuming tests, lot-based acceptance plans (AQL-based logic) can be appropriate—as long as it’s clearly defined and agreed as a lot acceptance method (not “allowing bad pieces to ship”).
Key Takeaways for Engineers and Buyers
If you’re building stainless steel products that must look a certain way and pass salt spray, here are the lessons we learned the hard way:
- Surface finish is not just cosmetic—it’s corrosion engineering.
- Blasting media choice is a critical parameter, not a shop preference.
- Passing a sample test isn’t the same as guaranteeing production consistency.
- EP and passivation do different jobs—know what risk each one addresses.
- When requirements evolve, you need a formal re-evaluation step to confirm that appearance and corrosion targets still coexist.
Final Thought
We share this case because it’s common in real projects: a customer wants a certain feel and look, then later needs a corrosion requirement, and the process ends up “patched” instead of engineered.
Our takeaway is simple:
Don’t just approve the appearance—approve the process boundaries that make the appearance and performance repeatable.
If your team is facing a similar conflict between finish requirements and corrosion testing, we’re happy to compare notes.