Recently, fabricators of structural steel for buildings have reported an apparent shortage of primer for structural steel. As such, some steel and coatings professionals have begun to ask questions regarding whether structural steel inside buildings even needs to be painted.
In May 2022, the American Institute of Steel Construction (AISC) answered these questions by issuing a Technical Advisory About Painting Interior Steel, which states that for almost 70 years, AISC has advised against painting or priming steel that will be enclosed by a building, finish coated with a contact-type fireproofing, or in contact with concrete. AISC’s reasoning for this is that painting or priming steel in these instances results in unnecessary costs, causes delays, and has a negative environmental impact.
In addition to this, Section M3.1 of the AISC Specification for Structural Steel Buildings indicates that shop paint is not required unless specified in the contract documents. “The surface condition of unpainted steel framing of long-standing buildings that have been demolished has been found to be unchanged from the time of its erection, except at isolated spots where leakage may have occurred,” the specification’s commentary notes. “Even in the presence of leakage, the shop coat is of minor influence.”
As you can imagine, AISC’s technical advisory caused some stir in the coatings community. To further clarify the issue at hand, we will briefly explain when it is appropriate to paint new steel, when steel will perform without a protective coating, and the limitations of primers used on steel framing for buildings.
Let’s begin with the fundamentals of why steel is coated at all. Carbon steel contains anodic and cathodic sites due to grain boundaries and the non-homogeneity of the steel chemistry. These sites are metallically connected, and electrons flow from the anode to the cathode in the presence of an electrolyte, such as water.
This results in a corrosion cell, deterioration of the anodic sites, and substrate deterioration. Left unchecked, this corrosion can result in section loss and potential structural deficiencies.
Further, hot-rolled carbon steel contains a mill scale that is cathodic to the base steel. When the thin, brittle layer of mill scale cracks (due to thermal expansion and contraction of the base metal, along with other factors), the available electrolyte contacts the base steel through breaches in the mill scale layer.
Because the mill scale is cathodic to the steel, the base steel becomes the anode, and deterioration is accelerated. Protective coating systems isolate the electrolyte from the base steel, effectively eliminating corrosion until the coating system is breached.
Thus, removing the mill scale (e.g., by abrasive blast cleaning) and applying a protective coating system makes sense when electrolytes are present in the service environment. But what about when they aren’t? Is it necessary to remove the mill scale and apply a primer and/or full coating system?
Arguably, that answer is no.
When Coating Systems Are (Not) Necessary
A coating system is defined as the preparation of the steel substrate (i.e., surface cleanliness and profile) followed by the application of one or more layers of a coating(s) that may possess barrier, inhibitive, and/or galvanic protection properties. While barrier properties may be in multiple coating layers, galvanic or inhibitive properties must be in the primer layer with a steel substrate because intimate contact with the material is paramount.
Coating systems are required for corrosion prevention when the service environment contains aggressive chemicals, electrolytes, and/or other conditions that would attack the steel and deteriorate it over time. This includes below- and above-grade pipes, water tanks, wastewater treatment facilities, bridge and highway structures, power-generating facilities, chemical plants, and non-climate-controlled spaces such as parking garages and hangars.
Coating of structural steel elements for the interior of climate-controlled buildings should only be done for aesthetics (e.g., architecturally exposed structural steel elements such as steel beams, trusses, columns, and/or diagonal bracing).
There is little, if any, value in removing the mill scale and applying a primer to steel that will be enclosed in a ceiling, wall, or other climate-controlled interior space that will not be visible. The mill scale will adequately protect the steel due to the absence of any appreciable electrolyte. Building costs are reduced, and because no coating is used, there is no contribution to volatile organic compound (VOC) emissions to the environment.
In addition, given the current state of coating material shortages, we shouldn’t be using a coating when it isn’t necessary. Having the steel “look good” during erection is an unnecessary cost and can be easily value-engineered out of a project with essentially no ramifications. Further, the coating may sustain mechanical damage during the transport, unload, and erection processes, increasing construction costs for touch-up work once the steel is on site.
To take it one step further, a primer alone should not be considered a sufficient coating system. The Free Dictionary defines a primer as “the first or preliminary coat of paint or size applied to a surface.” A primer is not a finished system and frequently does not possess the properties needed for long-term protection.
An alkyd or direct-to-metal (DTM) acrylic latex primer applied to steel and exposed to weather elements for a few months while waiting to be erected will likely show signs of deterioration. Building owners should not expect primers of this type to protect steel from corrosion, especially when the primer may be damaged during handling.
To Coat or Not to Coat?
As a project stakeholder, first consider whether painting structural steel elements is necessary. If it is, consider what surface preparation is required, what coating system will provide protection and/or aesthetic value, and what application method is preferred. Dipping certain elements, such as bar joists, is cost-effective but is unlikely to produce the desired finish and may result in extensive runs and drips. It may be best to shop-prime the steel, repair any damaged primer, then apply the midcoat and/or finish coat once the steel is erected.
Again, all this comes at a cost, so any steel elements in climate-controlled spaces that will not be architecturally exposed are probably better left uncoated.
This article was originally published in the November 2022 print issue of CoatingsPro Magazine. Republished with permission.