A Chemical Cleaning Conservation Management Plan

The Gateway Arch in St. Louis, Missouri, is a famous part of the Gateway Arch National Park situated on the Mississippi River’s bank. The stainless steel-clad catenary arch reaches 630 feet above the park grounds.

The Arch's stainless steel plates include a matte, satin directional finish and are roughly 60 inches tall by 144 inches wide. The Arch has been exposed to atmospheric pollution, Missouri weather, and the hands-on attention of park visitors since its completion on October 28^th, 1965.

The weathering and discoloration of the stainless steel skin caused by these factors became apparent by the turn of the 21^st century. In 2005, the first of several studies of the Arch's wear was carried out, and subsequent investigations and analyses continued through 2014.

The researchers discovered that the Arch is sturdy and that the kinds of corrosion and discoloration changed depending on where they occurred on the structure. This was because various construction techniques were used at different elevations.

The investigations concluded that, for aesthetic and corrosion mitigation purposes, the Gateway Arch should be cleaned, damage repaired, and some original welds addressed.

Background

Cleaning and Refinishing Study

The Association for Preservation Technology (APT), an interdisciplinary group that promotes the best practices and technologies for conserving and retaining historic structures, organized a specialist workshop in 2018 held at the monument in response to the study's findings.

Three cleaning methods — water pressure washing, chemical cleaning with scrubbing, and laser ablation cleaning — were used on the sample stainless steel panels and evaluated.

Each cleaning technique was assessed according to a set of standards, including gentleness, effectiveness, and economic viability. It was also necessary that the techniques did not block future cleaning methods.

Two examples of corrosion: (Top) Discoloration in the form of superficial corrosion staining to the stainless steel surface. This is a typical condition at the base and lower reaches of the monument. (Bottom) Corrosion associated with incised graffiti. The discoloration is from the corrosion of iron particles left by the implement used to scratch the graffiti into the surface or penetration through the passive film into the base metal. This is also common at the lowest two panels on each leg of the monument. Image Credit: Astro Pak Corporation

Tasks and Goals

The best cleaning technique would be the one most suitable for the substrate and circumstances, following the Secretary of the Interior’s Standards for the Treatment of Historic Buildings.

The cleaning aimed to eliminate any damage and discoloration evident on the surface while maintaining the design's quality, materials, craftsmanship, and aesthetics. A visual assessment would identify the solution resulting in a minimal deviation from a set of untouched control panels.

The Teams

The three teams invited to the Gateway Arch Expert Workshop were Astro Pak, Kärcher, and the collaboration between Evergreene Architectural Arts and Adapt Laser. Daryl Roll, Jordan Schaecher, and Timothy Velaquez were the agents for Astro Pak.

Attendees at the 2018 Gateway Arch Expert Workshop pose in front of the Arch. Image Credit: Astro Pak Corporation

The Technologies

Each team offered various strategies for achieving the surface restoration of the monument:

• Astro Pak — Chemical Cleaning — Pickling and passivating treatments were used to eliminate free iron and other pollutants from the surface and strengthen the passive layer of the stainless steel to provide better corrosion resistance.
• Kärcher — Dry Ice and Water Cleaning — Different temperatures and pressures of high-pressure water, steam, and dry ice (CO₂).
• Evergreene/Adapt — Laser Ablation Cleaning — A high-velocity supersonic laser and cold spray were used to deposit stainless steel while using a robot-mounted CO₂ laser to erase scratches and fill incisions and gaps.

Cleaning Methods: Performed on test panels. From left to right: Applying derouging compound (Astro Pak). Ultrahigh pressure (1000 bar) water cleaning (Kärcher). Laser cleaning (Evergreene/Adapt). Image Credit: Astro Pak Corporation

Challenges

The challenges were the same regardless of the technology:

• Any intervention should be reversible so that it does not limit future cleaning options.
• The ASTM A484/A484M No. 3 brushed finishes were applied to the panels in a factory setting before they were installed. However, some damage occurred during transit, and those areas had to be manually refinished on-site.
• The system chosen must be suitable for the substrate and the circumstances while preventing any harm or discoloration.
• Due to the design of the Arch, any solution had to work at different heights and angles.
• The teams had to take care of the many corrosive, damaged, and discolored conditions that exist:
  • Pollutants with chlorides close to the base
  • Graffiti at the base
  • Base-level superficial corrosion
  • Base damage from impacts
  • Depositions from the atmosphere on the entire structure

Examples of Damage and Discoloration. Image Credit: Astro Pak Corporation

The above images represent examples of damage and discoloration:

1. Oil deposits from visitors touching stainless steel.
2. Darkened line blemishes from straps used to handle panel during shipping.
3. Faint circular blemish caused by suction cups used to handle stainless steel panels during fabrication.
4. Blemishes where trussed strut was temporarily installed to stabilize legs during construction.
5. Incised graffiti
6. Damage caused by being struck with the claw of a hammer.

Process

The cleaning techniques were tested on ten replicas of the panels that had been soiled, deteriorated, and destroyed.

The four decades of pollution could not be perfectly replicated, but the panels were inscribed with ferrous metal objects, regularly sprayed with deicing chemicals, and then covered in steel filings before being repeatedly wetted to cause discoloration.

After another two weeks of exposure to the elements, the panels were given to the crews for cleaning. As none of the techniques were able to fill in incisions, it was decided that eliminating the engraved graffiti would not be one of the goals of the test. Three panels were given to each team, with the remaining ten left uncleaned as a control.

Astro Pak used a combination of different chemicals, application times, and techniques to prepare nine-panel sections.

To ascertain which procedure would yield the best results, technicians used a variety of the company’s unique chemistry, including AP401 and AP410 derouging gels, NeutraRouge^TM derouging solution, and its UltraPass^© passivation solution in various combinations.

The chemicals were applied by hand, spray, and brush, and scrubbing was done with an orbital polisher or by hand as well.

Cleaning Methods: The effectiveness of scrubbing by hand or with an orbital polisher were compared to each other as well as a method involving merely wiping away the compound after a period of time had passed. Image Credit: Astro Pak Corporation

Finished Panels: The three panels, designated (from left to right) 7, 8 and 9 following their cleaning using various methods. Image Credit: Astro Pak Corporation

Conclusion

Each team processed the panels and then provided them for inspection. Compared to a panel that had not been aged, they were first visually evaluated for how effectively they restored the panels’ aesthetic value.

The goal of the visual assessment was to identify the cleaning method that generated a look similar to the unaged control, and it was regarded as the most crucial component of the study.

There were also several assessments of the enterprises’ gentleness and economic viability.

The methods were also compared to the outcomes of an on-site cleaning experiment done in 2014 using cleansers and abrasives that were easily accessible on the market.

Water and Dry Ice Cleaning (Kärcher)

Pressurized water, steam, and dry ice removal techniques were found to be acceptable for removing surface deposits from the upper portions of the monument, but they left a discolored surface and were not suitable for removing surface corrosion or corrosion from iron deposits within the graffiti.

Laser Cleaning (Evergreene/Adapt Laser)

This method was appropriate for cleaning surface deposits from the monument's highest reaches, but there were worries that the climbing robot's wheels might leave streaks on the monument.

It was also unsuccessful in eliminating embedded metal oxides and only partially successful in removing surface corrosion. Several of the samples studied revealed variable outcomes, and, in some spots, the brushed surface seemed to have been destroyed by the lasers. Laser treatment is still regarded to have potential as technology advances.

Concerns were also raised concerning the surface banding that appeared due to laser pass overlap.

Chemical Cleaning (Astro Pak)

The study committee posted an official Conservation Management Plan, which stated, “the Astro Pak chemical cleaning was found to be successful in removing surface corrosion and embedded metal oxides within the graffiti. The chemical techniques that they utilized produced the appearance of the test panels closest to the control panel.”

The specular gloss aspect of the processed panels was also examined in the study and identified as “the most often used when the surface optical quality of a product needs to be evaluated.” The study noted that the panel that Astro Pak treated with NeutraRouge™, AP410 with hand buffing was similar to the control with “substantially smaller standard deviation.” 

After a visual inspection, each panel’s surface was checked using a profilometer to gauge its texture and a microscope to gauge any residual impurities. The surface of the steel cleaned by Astro Pak was closest to that of the panel that had not been processed or aged.

The chemical cleaning approach also proved the most effective and removed the deicing salt stains, but the water/dry ice and laser methods both left embedded iron particles and were therefore deemed unsuccessful.

The tests revealed that some solutions showed potential for addressing APT’s immediate goal of preventing the corrosion and degradation that are mostly brought on by deicing salts and etched graffiti on the lower sections of the Arch. While it is acknowledged that corrosion needs to be cleaned and the surface passivated to avoid premature aging, the structural integrity of the arch has not been damaged.

Chemically Cleaned: A micrograph of one Astro Pak sample shows that there were no embedded iron particles remaining. Image Credit: Astro Pak Corporation

This does not mean that APT plans to restore the surface to its original state when the monument first became operational. The association considers that the best long-term strategy for maintaining the monument and the integrity of its symbolism is to respect the natural aging process.

This led to a debate about maintaining graffiti as a sign of human interaction. The APT is also prepared to wait until the appropriate technology is developed to address the surfaces at the upper levels of the Arch. While the climbing robot demonstrations did not prove effective in removing corrosion, the analysis indicates that the technology might be a solution in the future.

Source: Astro Pak Corporation

This information has been sourced, reviewed, and adapted from materials provided by Astro Pak Corporation.

For more information on this source, please visit Astro Pak Corporation.