Coatings for Ductile Iron

Introduction

Ductile iron pipe (DIP) is widely used in a variety of applications, including water and sewer lines, particularly in diameters ranging from 24” to 48” (600mm to 1.2 m).

There has been considerable debate over the years as to how much external corrosion protection is required with DIP, which is somewhat less prone to attack than carbon steel. One industry association takes the view that polyethylene sleeves provide adequate exterior protection, but the generally accepted view is that a bonded coating such as polyurethane provides superior protection, especially in aggressive environments (such as corrosive soils or stray currents).

On the interior, a variety of technologies are offered and promoted by pipe manufacturers and coatings providers: melted-in-place polyethylene, cement mortar, filled epoxies and polyurethanes.

The Polyurethane Alternative

The effectiveness of traditional methods of corrosion protection has long been debated. Madison Chemical introduced the world’s first 100% solids polyurethane coatings and linings for steel back in 1981. Towards the end of that decade, consultants and engineers began to routinely specify these same coatings for a variety of DIP projects, as they provide excellent performance properties on all substrates – including high adhesion, abrasion resistance, impact strength, chemical resistance and undercutting resistance. They are also compatible with external cathodic protection.

A breakthrough in 2001 provided linings with excellent resistance to microbiologically induced corrosion (MIC) via a unique, permananent modification called MicroSpear. Where abrasion is anticipated to be unusually high, we have a ceramic reinforcement called NanoShield which significantly increases the abrasion resistance of these coatings by a factor of 3X. Hundreds of kilometers of DIP have successfully been lined and coated with Madison polyurethanes.

Application and Performance Advantages

Madison product ranges are very broad and, as such, offer a wide array of choices in terms of both application characteristics and performance attributes.

Some of the leading edge handling characteristics you can expect include ⁽¹⁾:

• Curing at virtually any temperature, winter and summer
• Non-flammable, solvent free and intrinsically safe
• Self-priming direct to metal, one coat systems
• High build application in a single multi-pass coat
• Rapid setting times (from seconds to hours)
• Choice of formats: Plural Component, Mix-and-Apply and PreCatalyzed
• Easy touch-up and repair
• D.O.T. rated as “non-hazardous”
• Excellent edge retention
• Surface tolerant

Important performance advantages include:

• Superior adhesion
• Excellent wet adhesion retention
• Resistance to chemical attack
• Resistance to gouging, abrasion and impact
• Flexibility and resistance to embrittlement
• UV stability
• Inert when cured; no risk of ‘tainting’ water supply
• Resistance to disbondment and undercutting
• Several national and international approvals
• Low water absorption
• Low permeance
• Special versions with anti-microbial and/or ceramic modified modifications for additional performance protection

(1) Not all the above features apply to every product