Molybdenum and its applications
Molybdenum Boosts the Corrosion Performance of Stainless Steel Handrails
By Gary Carinci, TMR Stainless (Consultants to IMOA)
Attractive appearance, low maintenance requirements, and increased safety have made exterior railings an important growth market for stainless steel. Type 316L stainless steel handrails, which contain 2% molybdenum, are the most cost-effective choice in many environments. Adding molybdenum to stainless steel significantly increases its corrosion resistance in salt (chloride) containing waters. In extremely aggressive coastal applications, where handrails are exposed directly to splashing with high-chloride brackish or sea water, stainless steels with more than 2% molybdenum are necessary to maintain a clean and rust-free appearance.
The Queens West Park handrails, seating, and lighting components are located on piers adjacent to the East River in New York City. The East River is a narrow, brackish tidal strait that separates Long Island from Manhattan Island. The glass bead-blasted Type 316L has performed extremely well in most of the applications (Figures 1 and 2). The exception is the splash zone. The splash zone is located where the waves, caused mainly by boat traffic, impinge on the pier wall and frequently splash onto the adjacent handrail section.
The handrails, seating, and lighting components located along the piers adjacent to the East River in New York City were fabricated using glass bead-blasted Type 316L.
The 316L bead-blasted railing components outside of the splash zone have maintained satisfactory corrosion resistance after six years of service.
The cyclic wetting and drying with brackish water produces an aggressive, corrosive environment due to the evaporative concentration of salt and chlorides on the stainless steel handrail. Within a few months after installation, stainless steel components in the splash zone were showing signs of corrosion (Figure 3). To aid in material selection for the additional stages of this project, Type 316L, AL 2003™, and Type 2205 stainless steel handrail samples with different surface finishes were exposed in the splash zone along the piers for approximately 15 months to assess their corrosion resistance (Table 1). The effects of both the stainless steel chemical composition and the performance of pickled, sand blasted, and mechanically polished surface finishes were assessed.
A reddish stain was evident on the 316L railing and panel adjacent to the pier wall, which is frequently splashed by the impinging waves caused by river vessel traffic. The evaporative concentration of salt and chlorides on the surface of the stainless steel results in an aggressive, corrosive environment.
After fifteen months exposure to the East River splash zone, the 2205 handrail samples displayed the best overall corrosion performance when compared to the S32003 and 316L samples (Figure 4). The 316L samples showed significant levels of surface attack for the polished, pickled, and blasted surface conditions. The S32003 samples displayed intermediate performance when compared to the 316L and 2205 samples in the pickled and blasted surface conditions. The 316L and S32003 polished samples appeared similar with a uniform stained appearance over the entire railing surface.
The pickled tube samples after fifteen months exposure in the splash zone displayed uniform, red staining on the 316L (top), light, localized staining on the S32003 (middle), and very limited stain spots on the 2205 (bottom) tube.
Pickling produced the optimum corrosion performance, while sand blasting slightly degraded the corrosion resistance of the samples. The mechanically polished samples displayed the highest levels of attack and staining when compared to the pickled and blasted surface conditions.
The Cost Savings
A brackish water splash zone is an aggressive environment due to the high salt (chloride) levels. Stainless steels with higher molybdenum contents are required to maintain a rust-free appearance. Type 2205 stainless steel with 3% molybdenum provides a maintenance cost savings over stainless steels with 2% molybdenum. Type 316L and S32003 would require more frequent cleaning to remove the salt deposits and corrosion stains.
AL 2003™ is a registered trademark of ATI Properties, Inc.
All photos taken by IMOA or its consultants.