Are Your ERW Pipes Suitable for Outdoor Structural Use?

Introduction: Beyond the Factory Finish

ERW (Electric Resistance Welded) pipes are commonly used in structural applications to provide everything from building columns to supports for trusses. But once these structures are outdoors, can you be sure a standard ERW pipe is suitable? Yes, with the right grade of material, and surface protection, very importantly.

Outdoors, steel faces constant corrosive attack from moisture, oxygen, and pollutants. Unprotected carbon steel will rust and jeopardize the integrity of the structure. This document will undertake an engineering analysis of the main factors that determine the suitability for outdoor application, and make a quantitative comparison of some of these protections.

1. Key Factors for Outdoor Suitability

Three primary factors dictate the performance and lifespan of an ERW pipe in an outdoor setting:

1. Material Grade:The underlying steel must have the appropriate mechanical properties (yield strength, tensile strength) to handle the design loads. Common structural grades like ASTM A53 or API 5L Grade B provide a solid foundation.
2. Surface Protection:This is the most critical factor. A protective coating is required to isolate the steel from the corrosive environment. The choice of coating directly impacts the pipe's service life and long-term cost.
3. Wall Thickness:A thicker wall provides a greater structural safety margin and a larger corrosion allowance, meaning it will take longer for any potential corrosion to compromise the pipe's load-bearing capacity.

2. Quantitative Comparison: Black Pipe vs. Hot-Dip Galvanized

The most common choice for outdoor structural applications is between standard "black" (uncoated) pipe with a paint finish and hot-dip galvanized pipe.

3. Welding Considerations for Galvanized ERW Pipe

When galvanized ERW pipes are utilized in structures where welding is to be performed in the field, you must consider a few factors. When welding, the process will burn off the barrier of zinc at the weld joint. To provide continued corrosion protection, the welded area will need to be repaired. The repair process typically will entail:

• Cleaning the weld: All welding slag and debris must be removed.
• Applying Zinc-Rich Paint: Using a high-quality cold galvanizing compound (a.k.a. paint with very high zinc material) will help replicate the barrier and provide some sacrificial protection.

The repair of the coating is very important as the weld should not become the point of failure for a structure due to internal localized corrosion.

Conclusion: An Investment in Durability

While standard black ERW pipe can be utilized outdoors without decreasing structural capacity (when painted), it will require ongoing maintenance to preserve its structural integrity. Hot-dip galvanized ERW pipe is the better engineering material for exterior applications that are long-term or structurally critical. The increased initial cost is justified by much longer service life (without maintenance). By specifying a galvanized finish you are specifying a product that has been specifically engineered to weather the outdoor elements as an engineering specification intends to help sustain the long-term safety and durability of the structure.