Understanding stainless steel pipe contamination and managing the risks

Myth: Stainless steel doesn’t corrode. That’s why they call it “stainless.”

Fact: It can corrode if it’s not handled properly.

Engineers in aerospace, semiconductor, biopharmaceutical and sanitary applications know the importance of preventing stainless steel contamination during regular use of a piping system.

Even so, stainless steel must be protected at every step in the long journey from origin to end use. That means that everyone who handles it —mills, distributors, fabricators, job site crews and all the shippers in between — is responsible for preventing contamination.

The whole supply chain must understand how it can happen, know what to do to prevent it and verify that others are being just as careful.

Sources of stainless steel contamination

Stainless steel owes its reputation (and its name) to chromium oxide, which forms naturally on its surface in ambient conditions.

Unlike flaky iron oxide, chromium oxide clings quite tightly to the surface, forming the protective barrier that makes stainless steel so resistant to corrosion. But this barrier can be overcome in a few different ways.

Exposure to chlorides

Strong chlorides can disrupt chromium oxide to form surface corrosion. If that’s not caught and addressed right away, this can evolve quickly into pitting corrosion.

Mitigating chloride exposure risk during normal use is relatively straightforward — just make sure you’re using the ideal stainless steel variant, or apply special coatings to boost resistance.

The greater challenge is avoiding inadvertent exposure prior to end use. Exposure pathways are numerous, but some examples include:

• Storage in improper conditions (for example, in coastal areas impacted by sea breeze)
• Uncovered shipment over roads where road salt was applied
• Accidental exposure to chloride-containing chemicals inside a facility or in transit

Exposure to ferritics

Contact with ferritic material can degrade chromium oxide, too. Preventing this is challenging due to the sheer number of potential sources of contact. If you can imagine it, it’s probably happened, but here are some general examples:

• Stainless steel was stored or shipped on racks that previously held carbon steel or other steel types
• Ferritic material was transferred to stainless steel from tools used to carry or process carbon or other steels
• Exposure via airborne ferritic particles (sparks, for example) generated during processing of carbon or other steels in close proximity to stainless steel

We’ve even observed stainless steel contamination from an interior weld inspection mirror that had once been inserted into a carbon steel pipe. Just once — that’s all it took.

Regardless of how the contamination is initiated, it can cause serious problems if it’s not addressed.

The pitting corrosion that often results from a contamination event is notoriously deceptive. Pits are practically invisible and most of the damage done by an attack occurs below the surface. Users realize there’s a problem only when they notice pinhole leaks or if valves don’t close all the way.

By that point, it’s likely that what started as local corrosion has spread throughout the whole system.