Stainless steel has contributed to one of Western Australia's architectural masterpieces overlooking the Murchison River Gorge.
Kalbarri National Park is located approximately 550km north of Perth in Western Australia (WA) and is dubbed as one of the state's best scenic nature destinations. Delivering a major tourism boost for the Mid-West region, the WA State Government with the Department of Biodiversity, Conservation and Attractions opened the highly anticipated Kalbarri Skywalk in June 2020.
The skywalk allows visitors to immerse themselves in breathtaking views of Murchison River Gorge and the natural beauty of the surrounding landscape. Two cantilevered viewing platforms 'float' mid-air 100m above the canyon, with one platform projecting 25m and the other 17m beyond the cliff face. Offering extensive 270-degree views from the highest point in the gorge system, the skywalk platforms were built and anchored into the 400-million-year-old sandstone.
The inspiration behind the skywalk's architecture and materials selection was to deliver a unique wilderness experience without detracting visitors from connecting with the natural environment. Designed by Eastman Poletti Sherwood Architects, the skywalk is 40km inland from the coastline and merges elegantly into the rocky gorge country and its geological features.
Built by Bocol Constructions, the skywalk and surrounding tourist infrastructure make extensive use of coloured patterned concrete and rust-coloured weathering steel to blend in with the surrounding landscape. Over 200m of grade 316 stainless steel handrails complement the structure delivering safety and stability for visitors.
ASSDA Member International Corrosion Services (ICS) was engaged to perform the chemical surface treatments on the stainless steel handrails, which included 128 units of 25mm rectangular hollow sections (RHS), ranging from 1100mm-1600mm in length. Fabricated and delivered by Boulevard Marine, the stainless steel material for the project was supplied by ASSDA Member Stirlings Performance Steels.
All sections underwent a post-fabrication, two-stage surface treatment process - chemical pickling, followed by electropolishing. Firstly, the sections were pickled and passivated in accordance with ASTM A380 Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems as a pre-electropolishing treatment. This involved immersion of the sections in chemical baths of up to 30 degrees Celsius to remove surface contaminants and heat-affected zones.
The sections were then dried, in preparation for the electropolishing process which was performed in accordance with ASTM B912 Standard Specification for Passivation of Stainless Steel Using Electropolishing. Each section was suspended by contact points from an anode and immersed in an electropolishing bath at a set current optimum corrosion resistance and to ensure an even and smooth finish (Ra value). The sections were then drained, carefully washed, air-dried, and individually wrapped to ensure no damage occurred during the six-hour on-road transport to Kalbarri National Park. All post-fabrication surface treatments were performed at the ICS workshop in Forrestdale.
A certificate of conformity was issued to the client by ICS to verify the completed treatments complied with the international standards specified. The pickling, passivation and electropolishing treatments were performed to ensure a high level of corrosion resistance was delivered, in addition to the material's durability, longevity, and low cost of ownership and maintenance. Electropolishing further delivered an aesthetically pleasing finish, complementing the structure's design.
Representative of a great design, the individual sections of the stainless steel handrails were bolted in place to avoid onsite welding and for future ease of maintenance. If any of the handrails require maintenance or replacement, the individual sections can simply be unbolted for offsite cleaning and surface treatment, and re-installed.
The skywalk attraction recognises the Nanda Aboriginal people and their heritage, as the traditional owners of the land. Interpretive information on all facets of the park's culture and natural history, including artwork created by several local indigenous artists adds to the Skywalk experience.
Technical Terms Explained
Pickling
The removal of any high-temperature scale and any adjacent low chromium layer of metal from the surface of stainless steel by chemical means. The process leaves a slight matt finish and provides a passive surface immediately upon rinsing, maximising the ongoing performance, durability, and corrosion resistance of stainless steel.
Passivation
The treatment of the surface of stainless steel, often with acid solutions (or gels), to remove contaminants and promote the formation of the passive film on a surface that was freshly created, e.g., through grinding, machining, or mechanical damage.
Electropolishing
A factory bath method of improving corrosion resistance and brightening the surface of components. It uses electrical current rather than chemical means to selectively remove peaks and reduce surface roughness to give a deep lustre. Electropolishing also improves corrosion resistance by thickening the protective passive film which delivers durability and performance in stainless steel for extended product life.
This article was featured in Australian Stainless Magazine Issue 77 (2023).