In industrial high-temperature applications, metals such as stainless steel (SS) and lower-chromium alloys are frequently insulated with materials containing alkali and/or alkaline earth metal oxides, particularly calcium: stone wool, glass wool, glass fibre blankets, needle-felt mats, microporous insulation, or calcium silicate. Some materials, such as AES wool or microporous insulation products, are processed into blanket form. Calcium-free microporous insulation materials often incorporate calcium-bearing glass fibre fabric coatings (e.g. E-glass), which, like directly applied AES mats, can bring chromium into contact with alkali or alkaline earth metal ions. These materials are used in sectors such as combined heat and power (CHP), engines, shipping, trucks and buses, as well as in the energy sector, turbine installations, incinerators, the process industry, and other applications where metal and insulation are exposed to elevated temperatures over extended periods. Under certain conditions, chromate compounds can form, i.e. Cr(VI). These chromate compounds are carcinogenic substances that can pose serious risks to human health and the environment. This article explains how Cr(VI) forms, what role insulation plays, at what temperatures the risk increases, and why it is important to remain alert even at lower temperatures.
Two mechanisms of Cr(VI) formation: solid phase and gas phase #
Based on recent studies (including Sayano et al. 2015 and Van Leeuwen et al. 2024), we distinguish two separate but partly overlapping mechanisms by which Cr(VI) can form in or on insulation material:
1. Solid-state reaction at the interface with the metal #
When chromium-bearing steels are in prolonged contact with calcium- or sodium-containing insulation materials, Cr can diffuse out of the steel surface. A Cr(III) oxide layer (chromium(III) oxide, Cr₂O₃) forms at the surface, which is then further oxidised to Cr(VI) under the influence of oxygen and metal oxides such as calcium oxide (CaO) or sodium oxide (Na₂O).
For example:
- Calcium oxide (CaO) + silicon dioxide (SiO₂) + chromium(III) oxide (Cr₂O₃) + oxygen (O₂) → calcium chromate (CaCrO₄) + silicon dioxide (SiO₂)
- Sodium oxide (Na₂O) + chromium(III) oxide (Cr₂O₃) + oxygen (O₂) → sodium chromate (Na₂CrO₄)
Although the original studies often cite calcium orthosilicate (Ca₂SiO₄) as a reagent, it is important to note that this specific compound does not occur directly in stone wool or glass fibre materials. These typically contain a mixture of calcium oxide and silicon dioxide in an amorphous or partially crystalline structure. On heating, these behave in a comparable manner and can participate in similar reactions that lead to the formation of stable Cr(VI) compounds such as calcium chromate.
Also relevant here: calcium oxide (CaO), a common component in insulation materials such as stone wool, glass fibre mats, and microporous insulation coatings, can react with atmospheric moisture to form calcium hydroxide (Ca(OH)₂). This calcium hydroxide is a strong base and can attack the passive oxide layer of stainless steel (the Cr₂O₃ layer), releasing more Cr(III), which can then be oxidised to Cr(VI) in the presence of oxygen. As a result, Cr(VI) formation may occur at temperatures from 250 °C, particularly under prolonged exposure.
2. Gas-phase formation and condensation of Cr(VI) #
At temperatures above approximately 500 °C, Cr(III) can volatilise as gaseous Cr(VI) species (such as chromium(VI) oxide, CrO₃, or chromium dioxide hydroxide, CrO₂(OH)). This vapour spreads with the airflow and condenses on cooler parts of the system or on insulation fibres. If the insulation contains elements such as calcium, sodium, potassium, or magnesium, these can react with the deposited Cr(VI) to form stable, solid chromates. Without these elements, Cr(VI) degrades back to Cr(III); in the presence of calcium or sodium, compounds such as calcium chromate (CaCrO₄) or sodium chromate (Na₂CrO₄) form.
This explains why chromate formation can also occur at locations with no direct metal contact, where Cr(VI) gases condense on or within the insulation. The complete process, from volatilisation to deposition and salt formation, has been experimentally confirmed in the study by Van Leeuwen et al. (2024).
Temperature: when does Cr(VI) form? #
Although solid-state reactions become dominant only from approximately 500–600 °C, evidence from both the literature and field practice indicates that Cr(VI) can form from around 250 °C:
- In Sayano’s study, Cr(VI) compounds were detected at test temperatures of 773–873 K (500–600 °C).
- Van Leeuwen demonstrates through thermodynamic calculations that stable chromates can theoretically exist from 100–250 °C, depending on chemical composition, moisture content, and duration.
- In the field, SEEF has found Cr(VI) on insulation exposed to temperatures between 250 and 400 °C, particularly where CaO-rich materials such as stone wool were used.
At SEEF, the working rule is: no lower limit can be excluded. Testing and monitoring is justified and prudent even in 250 °C applications.
Risks to health and the environment #
Cr(VI) compounds such as calcium chromate are:
- Carcinogenic on inhalation of dust (lung cancer, nasal tumours)
- Irritating and able to penetrate the skin on skin contact, because these chromate compounds are water-soluble. Full skin protection is required, including the face.
- Toxic to the environment: water-soluble and persistent
During maintenance work involving the removal of insulation, Cr(VI)-bearing dust can be released. This poses a significant risk to workers and creates legal and environmental liability. Because Cr(VI) binds to dust particles, it can spread readily through workspaces.
Detection and control #
SEEF’s patented test methods enable reliable detection of Cr(VI), including in complex matrices where standard methods (such as NEN 5617) fail. Comparative tests show that standard methods underestimate more than 90 % of the Cr(VI) present in calcium-bearing insulation, while the SEEF method maintains recovery above 80 %.
Control measures:
- Use calcium-free (alkali-free) insulation to structurally prevent chromate formation
- Install barrier layers between metal and insulation, for example a layer of calcium-free insulation
- Test regularly for Cr(VI) in applications at temperatures from 250 °C
- Use PPE and follow removal protocols during insulation inspections and remediation work
- Measure actual exposure during insulation inspections and remediation work
Conclusion #
Cr(VI) formation in insulation is a chemically substantiated and reproducible process that does not occur only at extreme temperatures. It arises via two routes:
- Solid-phase interaction between chromium and Ca/Na-bearing insulation
- Condensation of gaseous Cr(VI) on alkali-bearing surfaces
In both cases, the type of insulation material is a determining factor. Materials containing calcium, sodium, potassium, or magnesium carry an elevated risk of chromate formation, from as low as 250 °C.
SEEF supports you with detection, analysis, advice, and training, from investigation to risk management.
Want to know more? Contact us at info@seefbv.com or call +31 (0)85 047 05 74.
