Ductile Iron Couplings for Stainless Steel Pipes

Posted on January 14,

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Does Joining Stainless Steel Pipes with Ductile Iron Couplings Lead to Dissimilar Metal Corrosion?

Stainless steel pipes are favored in mechanical building services projects due to their strength, corrosion resistance, and low maintenance costs. However, the question arises: how should these pipes be connected? Is there a risk of dissimilar metal corrosion when using a grooved ductile iron coupling? The answer is no. This article will clarify how dissimilar metal corrosion develops and why using a grooved mechanical joining method is optimal for connecting stainless steel and copper piping systems.

Defining Dissimilar Metal Corrosion

Dissimilar Metal Corrosion occurs when an electrochemical process compromises the integrity of metals, diminishing their strength and thickness. This phenomenon is often referred to as Galvanic Corrosion or Bimetallic Corrosion.

Conditions Required for Galvanic Corrosion

1. Two different metals
2. An electrolyte
3. A return path for electric current

Understanding Dissimilar Metals

To comprehend dissimilar metals, one must reference the Galvanic Series, which ranks metals according to their nobility or resistance to chemical reactions.

The Galvanic Series Chart ranks metals based on their nobility. The more noble metal, positioned higher on the chart, is the cathode. Conversely, the less noble metal, found lower on the chart, is labeled the anode. Cathodic metals possess a higher relative electrical potential compared to anodic metals.

Severe attacks typically occur between dissimilar metals with significant differences in their relative potentials. For instance, titanium and aluminum would face much greater corrosion risk compared to copper and brass due to their varying positions on the Galvanic Series chart.

Role of Electrolytes in Metal Corrosion

To understand the mechanisms behind attacks on dissimilar metals, it's essential to consider the flow of ions between them.

Each metal possesses a specific relative electric potential. When metals with different potentials make contact in an electrolyte's presence, a low-energy electrical current flows from the anodic metal to the cathodic metal. As mentioned earlier, more noble metals are cathodic, while anodic metals have a higher likelihood of corroding when connected to a cathodic metal.

Creating a Corrosion Pathway

When dissimilar metals touch in a corrosive environment, the anodic metal experiences galvanic corrosion while the cathodic metal enjoys galvanic protection. An electrolyte facilitates ion migration from the anode to the cathode, leading to accelerated corrosion of the anodic metal. Meanwhile, the cathodic metal may corrode at a significantly slower rate or not at all.

Consider two improperly joined metallic pipes made of different metals with water flowing through them. The anodic metal here is lesser noble, and the electrons travel from this metal to the cathodic one via the water, acting as the electrolyte. Ultimately, the anodic metal is likely to corrode over time due to the inadequate insulation between the two metal types.

Surprisingly, situations frequently occur where dissimilar metals touch yet corrosion remains absent. A simple example is carrying a copper penny and a nickel together. Based on the Galvanic Series, copper is more anodic than nickel; thus, although they are dissimilar, neither experiences corrosion due to the absence of an electrolyte and a path.

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This same principle applies to Victaulic products in building service projects. Many applications necessitate using stainless steel pipes, renowned for their strength, corrosion resistance, and overall low maintenance costs. Although they are dissimilar metals, ductile iron housing couplings frequently emerge as the best choice for connecting stainless steel pipes.

Utilizing Victaulic couplings with ductile iron housings on stainless steel is standard practice when pipe materials are selected based on their compatibility with the fluid medium and where external corrosion risks are minimal. Thanks to the unique design of the coupling housings and the elastomer gasket, the housings will never touch the internal fluid medium. The pressure-responsive gasket between the joint creates a leak-tight seal, isolating the fluid medium from contact with the coupling housings.

Victaulic boasts extensive on-site experience with galvanized ductile iron housings installed on stainless steel piping systems without reported cases of galvanic corrosion between pipes and couplings.

What Happens in Corrosive Environments?