Poison Pads (also called wear pads in some contexts) are stainless steel plates or pads used in stainless steel fabrication and tank/pipe construction to prevent metallurgical contamination from carbon steel.
Why Are They Called “Poison Pads”?
Carbon steel can “poison” stainless steel by transferring iron particles or embedding during welding, grinding, or contact. This contamination leads to rusting or reduced corrosion resistance on the stainless steel (often called “iron contamination” or “free iron”). The stainless steel pad acts as a protective barrier so the carbon steel doesn’t directly affect the main stainless component.
Main Uses in Stainless Steel Fabrication
- Between Carbon Steel Supports/Attachments and Stainless Steel Tanks/Pipes
- Carbon steel legs, brackets, channels, or pipe hangers are welded to the point pad.
- The poison pad (stainless) is then welded to the stainless steel vessel or pipe.
- This isolates the stainless material from carbon steel contamination.
- Wear Protection
- Used where stainless pipe or tube is supported by carbon steel hangers/Supports to prevent wear and contamination.
- Tooling and Fixturing
- Sometimes placed under tools or key plates during fabrication to avoid carbon impregnation from carbon steel tools/fixtures.
Typical Specifications
- Material: Usually 304 stainless steel (sometimes 316).
- Thickness: Commonly 7 gauge (~0.187″) or 3/16″ thick.
- Size: Made larger than the carbon steel part being attached.
- Design: Often include a weep hole (like reinforcement pads) for pressure testing or to detect leaks.
Why They’re Important
Stainless steel relies on a passive chromium oxide layer for corrosion resistance. Carbon steel contamination breaks this layer and causes surface rust, which can spread and compromise the entire fabrication — especially in food, pharmaceutical, chemical, or outdoor applications.
Poison pads are a cost-effective way to mix carbon steel supports (cheaper and stronger for structural needs) with stainless steel product-contact surfaces without ruining the stainless properties.
In short, they’re protective barriers nicknamed “poison pads” because they stop carbon steel from poisoning the stainless steel’s corrosion resistance.
Welding poison pads (also called wear pads) in stainless steel fabrication is a two-part process that protects the main stainless steel vessel, tank, or pipe from carbon steel contamination while allowing cheaper/stronger carbon steel supports or attachments.
The poison pad itself is stainless steel (usually the same grade as the vessel, like 304L or 316L). It acts as a sacrificial barrier: you weld the pad to the stainless component first (SS-to-SS weld), then weld the carbon steel part to the pad (dissimilar weld using special filler).
Quick Tips for Success
- Always use dedicated stainless fabrication practices (separate tools, benches, or temporary poison sheets).
- For pipe supports: The wear/poison pad is welded to the stainless pipe; the carbon hanger simply rests against or is clamped to the pad.
- On thicker stainless (¼” or more), poison pads are sometimes omitted, but they’re still recommended for best practice.
- Follow any project specs or codes (ASME, etc.) — many require this exact dissimilar-welding approach.
This method is standard in tank, vessel, and pipe fabrication shops and is explicitly called out in many engineering specifications for stainless systems. If you’re working on a specific grade (e.g., 316L) or code job, double-check the exact filler and qualification requirements.
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