Corrosion of Stainless Steel Handrail caused by Fabrication Process
ENVIRONMENT: Urban office building
EQUIPMENT: Pedestrian handrail
MATERIAL: Type 304 Stainless Steel
SERVICE TIME: 8 months
The handrail and balusters (referred to hereafter as the “handrail”) are located on a handicap ramp at an office building. The handrail is out of doors, but, along with the ramp, is covered by a canopy. The building and handrail are new, being less than 8 months old. Within approximately 4 months of its installation, the handrail has developed water spots and a light-to-medium rust-like appearance. The handrail is reported to be fabricated from Type 304 stainless steel, which would normally be expected to resist corrosion in atmospheric exposures. Reportedly, the handrail has been cleaned by polishing with steel wool, but the rust-like discoloration has returned. An on-site contractor has exposed a piece of stainless steel from the same lot of material as the handrail at a distance from the handicap ramp, and an identical rust-like discoloration developed, indicating that the cause of the corrosion is not specific to the ramp location.
A representative of Corrosion Testing Laboratories, Inc. (CTL) made a site visit to the office building to examine and photograph the handrail and surrounding environment, and to collect samples of the rust by stripping with replication tape (a non-destructive technique). Along with the site visit, the on-site contractor to aid in the investigation supplied the piece of stainless steel exposed by the on-site contractor, referred to above, to CTL.
Investigation of the cause of the rusty appearance of the handrail included:
On Site Examination
During the on-site examination, the CTL representative met with company representative, who escorted him around the office building, pointing out the discolored handrails. There were two (2) sets of handrails: one set at the entrance of the building and the other adjacent to a fire exit. A close-up of the entrance railing showing the red/brown staining and rust-like patches is seen in Figure 1.
The entrance handrail is on the east side of the building and is enclosed to the north and west. The handrail by the fire exit is on the south side of the building and is partly enclosed to the north. As mentioned in the Background, the handrails and walkways are open to the atmosphere but covered under a canopy. The setting is within an urban residential neighborhood, however there is a train maintenance yard across the street from the main entrance to the building, about 0.2 miles west from of the office building.
The entrance handrail appeared to be more stained and affected than the fire exit handrail, owing to the effect of lighting and contrast. The differences between the two were minor from a total affected surface area aspect. Equal rust staining of the concrete around the vertical balusters at the two locations was observed.
Knife scrapings, with a stainless steel blade, were collected, as well as replication tape stripping of rusted areas. Photographs were taken of the handrails and the surrounding areas. The company representative provided Mill test reports of the stainless steel used in the fabrication of the handrails.
Visual and Microscopic Examination of Supplied Sample of Stainless Steel
The received sample of stainless steel was of a rectangular cross-section, approximately 11” long x 2” wide x 1” thick, with 5/16” hole drilled at each end.
The sample had been mechanically finished on all faces by an unknown means using a fine abrasive (roughly 240 grit or finer). Immediately evident on the sample was a brown rust-like streak running lengthwise roughly down the center of the thickness dimension on both such faces (Figure 2). Accompanying the brown streaks were numerous surface defects oriented perpendicularly to the lengthwise direction of the brown streak, as well as sweeping grind marks oriented approximately 45 degrees (Figure 3) to the lengthwise orientation of the grinding marks outside the area with the brown streak. The defects had the appearance of laps created by mechanical working of the metal surface. Other surface defects were readily visible on the sample, some of which were accompanied by a rust-like product (Figure 4).
Under the microscope, the brown streaks had a superficial appearance as if they were dirt on the surface of the stainless steel sample rather than the result of corrosion of the stainless steel. Several brown, rust-like spots were associated with small particles stuck to the surface of the sample.
Energy dispersive spectroscopy (EDS) was used to obtain a semi-quantitative elemental analysis of the various rust-like products obtained on-site and in-house by stripping with replication tape. A small piece was cut from the supplied stainless steel sample (in-house) was also analyzed by EDS. The results were as follows:
Note: In an EDS spectrum, sulfur and molybdenum peaks overlap; for this reason, since the analysis for only sulfur was performed, the sulfur content of the base metal appears higher than is allowed by specifications for Type 304/304L stainless steel.
Oxalic Acid Cleaning
An approximately 1 inch long piece was cut from one end of the supplied stainless steel sample. Using a toothbrush and a 10% solution of oxalic acid in de-ionized water, the brown streak (determined from the EDS to be mostly iron) was removed from the piece of stainless steel (Figure 5). Examination of the stainless steel underlying the brown streak showed a residual gray discoloration of the surface of the stainless steel, but no corrosion, as would be evidenced by dulling and/or loss of relief on the grinding marks.
The cleaned piece of stainless steel was placed into tap water at room temperature and allowed to sit, undisturbed, overnight. The next day, a few brown spots were visible in the vicinity of the gray discoloration noted above. The piece was again cleaned with oxalic acid and replaced into tap water for approximately 24 hours. The next day, a few brown spots were again observed on the piece. The piece was cleaned a third time and placed into tap water overnight. After this third cleaning cycle and immersion in water, no rust appeared, and the piece of stainless steel appeared to be thoroughly cleaned of the iron contamination. However, the gray discoloration remained, and appeared to be the result of either excessive heating of the stainless steel from grinding, or some constituent of the grinding tool.
A small piece was cut from the received stainless steel sample and prepared metallographically, using standard techniques, so that a few surface defects (noted under Visual and Microscopic Examination, etc., above) could be observed in cross-section and also so that the microstructure of the stainless steel could be evaluated. The prepared metallographic sample was etched using an electrolytic 10% oxalic acid etch.
The surface defects were shallow, ranging from 0.03 mm to 0.05 mm (1.2 to 2 mils) in depth, and were accompanied by intergranular attack (IGA) of the stainless steel to a depth of approximately 1 grain (Figure 6).
The etched metallographic sample showed a normal equiaxed twinned austenitic grain structure (Figure 8) for Type 304/304L stainless steel, with a grain size of approximately ASTM No. 6. The surface of the piece (with the brown streak) showed, in cross-section, deformation of the grains indicating mechanical working, as by grinding (Figure 8). Laps at the surface were also observed (Figure 7).
Type 304/304L stainless steel is a “work-horse” grade of stainless steel frequently used in architectural applications, as well as in cookware, appliances, and numerous other applications in which its resistance to corrosion in a wide range of environmental conditions can be utilized.
Under the environmental conditions that we would assume to exist on a handicap ramp at a health facility, Type 304/304L stainless steel would be expected to perform satisfactorily as a material of construction for a handrail. Type 304/304L stainless steel, being an alloy of iron, chrome, and nickel, would not rust in the manner of a non-stainless ferrous alloy (“steel”) when exposed to normal atmospheric conditions. In the absence of any surface contamination, it would be expected that water-spotting, resulting from the evaporation of rain or cleaning water, would be the only cosmetic problem experienced with a Type 304/304L stainless steel handrail exposed out-of-doors.
EDS analysis of the supplied stainless steel sample shows that the sample was Type 304/304L stainless steel, and that the proper material was supplied for the handrails. The metallographic analysis indicated that the sample is in a satisfactory metallurgical condition. (It should be noted that information supplied by the office building shows that three heats of 1” x 2” flat bar stock and two heats of ˝” diameter round bar stock were used to fabricate the handrails. Our analysis of the composition and metallurgical condition of the handrail was performed on only one heat of 1” x 2” flat bar stock.)
The results of the EDS analysis of the rust-like products stripped from the handrail on-site and from the supplied sample of stainless steel indicate that the products are composed of iron or “dirt” (silicon, calcium, magnesium). The fact that the products are composed mainly of iron indicates that the patchy rust-like appearance of the handrail is in fact the result of rusting (oxidation) of the iron present as contamination on the surface of the handrail.
The iron contamination on the Type 304/304L stainless steel handrail is most likely the result of mechanical processing of the handrail components, either by grinding or brushing during finishing, with steel or contaminated stainless steel abrasives or brushe, which was supported by the brown rust streaks with the laps and mechanical deformation of the metal surface. On the same sample, rusting associated with obvious mechanical damage (Figure 5) to the sample surface lends additional support.
A question that arises is: When did the contamination occur? The fact that the entire handrail is affected, and that five lots of Type 304/304L stainless steel are involved, points to the handrail fabrication process as being the point at which contamination occurred, rather than at an earlier point. Also, the fabricator presumably applied the non-mill finish of the surface of the supplied sample of stainless steel and the handrail, which is in fact contaminated.