The advantages of intumescent coatings in protecting structural steel from corrosion and fire

By Roger Soler Palau, Group Product Manager Cellulosic PFP, Hempel A/S

Increasingly, construction projects are turning to steel as their core component. Steel enables strong and resilient buildings with wider spans and more open areas to be built more quickly, economically and with a reduced environmental footprint. Protecting steel framed buildings against fire is a vital part of the planning and construction process and safety features are generally a mix of active and passive mechanisms. When fire or smoke is detected, active protection will act to suppress or slow the smoke and flames using equipment such as fire-doors, sprinklers and alarms. Passive fire protection (PFP) is generally implemented to protect the building itself by insulating the steel structure to maintain the structural integrity of the building for as long as possible.

Fires behave differently depending on the fuel source and they tend to fall into two distinct categories. Hydrocarbon fires are fuelled by flammable liquids such as petrol, diesel or another oil derivative and are generally associated with the oil & gas industry. Cellulosic fires are caused by the burning of furniture, wood or paper and usually occur within civil infrastructure such as residential and commercial buildings. Hydrocarbon fires will reach high temperatures much more quickly than those of a cellulosic nature but, even so, a cellulosic fire will reach 500˚C in as little as three minutes and then escalate to more 1000˚C shortly afterwards. Temperature versus time charts are well established and standardised for both types of fire and these are used across the world to specify the required protection.

Safeguarding the structure
When the temperature of steel approaches 400 - 550˚C, its strength and stiffness significantly reduces and this can have a devastating impact on a building that relies on steel for its structural stability. The main role of passive fire protection is to insulate the structural steel from heat and to prevent it – for a specified period of time – from reaching the Critical Core Temperature where the strength of the steel element will not be sufficient to support the load. Achieving this will effectively “buy time” for emergency services to attend; for safe evacuation of the building; and for fire-fighters to extinguish the fire.  In short, these passive protection measures will minimise damage, reduce losses and potentially save lives.

For cellulosic fires, structural steel members may be insulated by using fire protection materials such as boards, inorganic sprays and intumescent coatings. Each type of solution has its “pros and cons” depending on the nature of the project but all of them are tested and certified against a range of international standards which differ depending on the individual country. Standards used, for example, include UL 263 for the USA, EN 13381-8 for Europe and BS-476 21/22 for UK and British related countries.

To meet these standards, the steel usually needs to retain around 50 per cent of its structural strength (depending on the standard) for the specified length of time under standardised cellulosic fire conditions.

Intumescent coatings are very widely used as passive fire protection for structural steel due to a range of advantages which include:

  • Good aesthetic finish and adaptability to the shape of steel, this enables the steel to be exposed as well as protected.
  • Ease of application, this reduces the application time and associated cost as well as the likelihood of defects.
  • Coatings are applied as a compatible system of primer+intumescent+topcoat, all them tested together for fire and corrosion protection.
  • Mechanical strength to avoid impact damage.
  • Flexible to adjust to structural movements such as vibrations, expansion and contraction caused by temperature differences and load changes.

Intumescent paints are applied to steel in thin layers and are inert at the normal operating temperature of a building. Once the temperature starts to rise above 200-250˚C, the coating will start expanding to many times its original thickness (typically around fifty times) to insulate the steel beneath. The swelling is a result of a complex chemical reaction within the coating that comprises a mix of binder system, catalysts, carbon source, inorganic pigments and blowing agents. Together, this creates an expanded char that incorporates millions of microscopic gas cavities within an inorganic and carbonaceous matrix. In effect, the coating morphs into an expanded layer with low thermal conductivity properties that insulates the steel beneath.

The period of time that a coating is required to protect the steel is defined in the building code of most countries and includes parameters such as the building size, type of use and the estimated evacuation time. Fire protection usually ranges from 15 to 120 minutes but can be much longer – up to 4 hours in some situations. The longer the protection time, the higher the dry film thickness (DFT) of the intumescent layer is required to protect the steel from reaching the Critical Core Temperature (where strength will be compromised). Hempafire Pro 315 – the widely proven intumescent product from worldwide coatings manufacturer, Hempel – has been certified according EN-13318 and BS-476 standards to provide up to 90 minutes protection for cellulosic fires. It also exhibits outstanding results in thickness efficiency and coverage in fire durations of 60 minutes, this is the length of time that the majority of the buildings need to be protected.

Protecting the steel from corrosion and fire
Passive fire protection intumescent coatings must protect against fire, but they must also protect the steel against corrosion in a range of environments. For this to be effective, it is important that the PFP coating system is made up of compatible and tested primers and topcoat sealers. The PFP system will include the following layers, each of them important:

  • the primer to protect against corrosion of the steel but also to enable the adhesion of the char in a fire scenario;
  • the intumescent to deliver the char with thermal insulation for the specified fire duration times;
  • the topcoat sealer to give the decorative finish and colour and to protect the steel from corrosion and the intumescent layer from weathering.

In more aggressive corrosive environments the coating system will be specified for higher thicknesses of primer and topcoat and the chemistries of the products used will differ. Intumescent paints usually fall within three main blocks of chemistries which define their suitability.

  • 1K water-borne products (usually acrylic) provide very good thermal insulation efficiency versus thickness applied. But as they are very sensitive to water, most intumescent manufacturers do not recommend their use in exterior or above C3 conditions.
  • 1K solvent-borne products (also usually acrylic) can provide very good thermal insulation efficiency by thickness applied, and their chemistry allows their use in exterior more demanding corrosive environments up to C4. They tend to be easy to apply, fast drying and give excellent aesthetic results.
  • Multi-component products (epoxy or other hybrid technologies) have been introduced in recent years and can provide corrosion resistance in C5 environments. Whilst they exhibit fast drying times, they tend to be more complex to apply and require higher DFTs using special equipment, trained crews.

A system incorporating Hempafire Pro 315, for example, can be applied up to the high corrosive C4 exterior conditions making it suitable for most urban and industrial areas. As these paints are applied inside residential and commercial buildings, their aesthetic appearance must be of good quality resulting in a smooth finish that reduces the need for cleaning and maintenance. Hempafire Pro 315 dries with an even and smooth surface (with no orange peel or other defects) to enhance the appearance of the exposed steel and displaying the contemporary look that is seen in many modern airports, stadiums and other landmark buildings.

Hempafire Pro 315 delivers a higher thermal efficiency per applied thickness than most similar products and so can give the specified fire protection with thinner DFTs, thus reducing the amount of paint used to between 4% to 20% of similar products in the market. This reduces application cost and drying times, as the thinner PFP layers dry faster and can be applied in fewer coats. This means that the steel can be handled or overcoated much sooner and efficiency is optimised.

Off-site application is increasingly demanded by those contractors who want to receive the steel already coated and shorten the construction phase. The painting operation is performed efficiently and in a more controlled manner but the time to deliver the protected steel becomes a key driver to maximize the throughput of the paint-shop. As Hempafire Pro 315 requires thinner layers while easy to apply, it dries faster and can be overcoated and handled more rapidly. This allows the off-site contractor to optimise costs, throughput and turn-around times.

When used in combination with other Hempel products, the entire protection system – primer, intumescent and top-coat – can be applied in less than 24 hours and can be ready for use within two days or less depending on the specific project conditions.

For those applications at the building itself (on site), contractors are seeking to reduce the amount of coats to apply, thus reducing application costs and also the time to finish the job. Hempafire Pro 315 is a high build coating – up to 1,600 microns DFT in one coat can be applied without sagging, this means that almost all steel sections can be protected with just one coat. 

Selecting a passive fire protective coating is complex. The paint must protect the steel from corrosion in its particular environment, be easy to apply and result in a high aesthetic finish. But above all, it must protect the underlying steel from deteriorating when impacted by the heat of a fire. Using an expert is always helpful to ensure the optimum choice is made; ensuring that cost and application efficiencies are recognised; and the final result is pleasing to look at. Hempel offers this service across its global network and also from its new state-of-the-art fire protection laboratory in Barcelona where teams of scientists, technicians and applicators continue to innovate and develop increasingly effective coatings.

Using steel in the construction of a range of buildings is becoming increasingly common. Paint manufacturers, such as Hempel, continue to research and develop innovative coatings that adequately protect the steel in the event of a fire. And this prolongs the structural integrity of the building and potentially saves lives.