Questions and answers on chromate formation
The answers are taken from the expert analysis "The Overlooked Ingredient". The section reference after each answer points to the section where the statement stands with its sources.
Formation pathway and conditions
How does chromium(VI) form on an insulated hot component?
High temperature insulation contains basic oxides, above all calcium oxide and sodium oxide. Above about 300 °C they react with the chromium oxide layer of chromium alloyed hot components to form calcium chromate and sodium chromate; sodium chromate already from about 200 °C.
Which four conditions does the reaction need?
The reaction needs a chromium source, a basic oxide, oxygen and temperature. All four conditions are present in an insulated hot plant during normal operation.
Is the chromate already present in the insulation material when it is delivered?
No. In the delivered state no chromate is present; it forms at the interface between insulation and component and forms again after every start up.
When is the chromate released?
It is released during maintenance, removal, cleaning, overhaul and dismantling, as dust that leaves the work area on tools, clothing, gloves and removed components.
Is a calcium content stated as CaO enough for the reaction to proceed?
A calcium content stated as CaO in glass or silicate does not automatically mean that free calcium oxide is present. Reactive availability is decisive for the application-related assessment. Calcium silicates can also supply calcium for chromate formation.
What role does sodium play?
Sodium is a constituent that is effective in process-chemical terms and can promote and control chromate formation. Soda accelerates and controls the oxidation to chromate.
Does this affect stainless steel only?
The reaction requires a chromium source, as a rule the passive chromium oxide layer of chromium alloyed hot components.
What is documented
How long has the formation pathway been known?
The formation pathway has been described in patent form since 1939. United States patent 2,176,687 expressly claims the production of calcium chromate and sodium chromate from a chromium source, lime or soda and oxygen.
Is that only in the patent description or also in the claims?
Claims 22 and 25 name a mixture of calcium chromite and calcium chromate; claims 23 and 26 additionally name sodium chromate. The chromates are thus not merely mentioned in the description but are part of the claimed technical process.
Is there experimental evidence at the metal to insulation interface?
Yes. For SUS304 and calcium-silicate insulation material, 160 mg Cr(VI) per m² of contact surface was measured after 1,000 hours at 873 K (approximately 600 °C).
How does the amount develop over time?
After 5,000 hours at 873 K, the amount of Cr(VI) in the uncoated SUS304/Ca-Si-O system was 940 mg/m²; with the additional calcium-containing, self-hardening coating insulation material it was 3,890 mg/m².
Can the formation be prevented by a barrier layer?
A barrier layer 3 to 4 µm thick prevented formation almost completely.
When did a manufacturer first warn about it?
In April 1990. The Babcock & Wilcox Power Generation Group issued Plant Service Bulletin PSB-43 to owners and operators of boilers and steam generators. The first version concerned chromium-containing refractories; the revised version PSB-43A of March 1994 expressly extends the scope to “plastics, insulation, and textiles”.
Does that still appear in safety data sheets today?
On the safety data sheets of its chromium-containing refractory products HarbisonWalker International carries the sentence: “The chrome compounds (Cr III) in this product may be altered to a hexavalent compound (Cr VI) under certain use conditions, such as exposure to alkali salts and/or high temperatures.” It was checked on four product sheets from the years 2018 to 2021.
What does the analysis rest on besides the literature?
On our own site inspections and wipe and material samples taken since 2019 on engines, power plants, incineration plants, ships, vehicles and construction machinery.
Insulation materials and their basic oxides
Which insulation materials carry the reaction partners?
The appendix to the expert analysis lists twenty material rows with documented content of basic oxides, from E-glass through mineral wool, AES wool, ceramic fibre, calcium silicate, perlite and vermiculite to the assembly paste.
How much calcium oxide is in E-glass?
CaO 16 to 25 per cent, MgO 0 to 5 per cent, Na₂O and K₂O 0 to 2 per cent according to the standard for glass fibre strands.
How is mineral wool delimited from ceramic fibre?
By the same oxide content: more than 18 per cent by weight of sodium, potassium, calcium, magnesium and barium oxide together leads to mineral wool, up to and including 18 per cent by weight to ceramic fibre. The content of basic oxides is therefore the characteristic that distinguishes the two substance groups.
Does an aerogel blanket contain calcium?
In the safety data sheet no calcium compound and no sodium compound is declared; more than 50 per cent textile glass without chemical specification is stated. The data sheet of the blanket rated to 650 °C names a fibre batting, the older version a glass fibre batting; no oxide analysis of this glass appears in any of the sheets. Blankets rated to about 650 °C carry an E-glass component, seen in the field, for which the standard gives CaO 16 to 25 per cent.
Is ECR glass less harmful than E-glass because it is boron-free?
The standard defines ECR only as a boron-free modification of E-glass and publishes no oxide table of its own. A comparison of the two glass types gives CaO 16 to 25 per cent for boron-containing E-glass and 17 to 25 per cent for boron-free glass; in the direct comparison in the same work the boron-free glass reached CaO 22.3 to 22.9 per cent against 19.55 to 22.35 per cent for the borosilicate glass.
What is the composition of basalt fibre?
The manufacturer publishes no oxide breakdown. Publicly available figures give SiO₂ 53 per cent, Al₂O₃ 18 per cent, Fe₂O₃ 10 per cent, CaO 9 per cent, MgO 5 per cent, Na₂O 3 per cent, K₂O 1 per cent and TiO₂ 1 per cent. A review gives for basalt rock CaO 8.59 per cent, MgO 4.63 per cent, Na₂O 3.34 per cent, K₂O 1.46 per cent and Cr₂O₃ 0.06 per cent.
Why is calcium not a line of its own in the safety data sheet?
The safety data sheet describes the delivered state and carries no meaning for the assessment of a maintenance operation. In the data sheets of the fibre products calcium does not appear as a line of its own, although it is contained in the substance definition behind the CAS number.
Are there substitute materials without these oxides?
In high temperature insulation substitute materials without alkali and alkaline earth metal oxides are available; the substitution check can therefore be carried out with a result.
Analysis and why a negative result proves nothing
Is there a standardised method for measuring chromium(VI) in insulation materials?
No. No standardised method exists for this matrix; there is neither a NEN nor an ISO nor a NIOSH method tailored to insulation materials. ISO 15192:2021 is intended for waste and soil.
Why is the matrix the problem?
Because the matrix is itself the reaction partner: rich in calcium, basic and, once in service, permeated with soot, oil and corrosion products. According to the laboratory concerned with it, transferred procedures give results that are too low by 20 to 30 per cent in mild cases and values close to zero in strongly calcium-containing samples.
How large can the error become?
In a matrix with 16 per cent zinc the recovery rate under NEN 5617 was 0.2 per cent, under a further accredited method 6.5 per cent and under the method of SEEF B.V. 35.3 per cent.
What must a test report state for it to mean anything?
Method, matrix and recovery rate in that matrix belong in the report, not only the numerical value. An analytical result for chromium(VI) without a statement of the recovery rate in the same matrix says nothing about the content.
What does the American method require?
The EPA method requires spiked samples before digestion, soluble and insoluble, with an acceptance range for the recovery rate of 75 to 125 per cent; outside that range the entire batch must be repeated.
Does a negative result prove absence?
As long as the recovery rate in this matrix is not demonstrated, a negative result is no proof of absence. That is why the German TRGS 561 does not rely on a measurement but on the presumption of presence.
Is a colorimetric spot test suitable for clearance?
A French field study found 869 ng Cr(VI) at the turbocharger while the colorimetric spot test at the same point gave a negative result. The authors note that several positive surface samples were not detected by visual colorimetric tests, and they expressly conclude that these tests are to be used with caution.
Limits of subsequent treatment
Can chromium(VI) be rendered harmless afterwards?
The reduction of Chromium(VI) to Chromium(III) is chemically possible and requires a suitable reducing agent, a suitable pH range, sufficient contact time and proof of effectiveness.
Are these conditions met in practice?
The work instructions available do not state the conditions that the certificate of the testing laboratory expressly names, and proof of effectiveness for the insulation matrix does not exist.
Does penetrating oil reduce chromium(VI)?
No. For the penetrating oil, the same laboratory that certified the effectiveness of the neutralising agent subject to conditions has refuted any reducing effect: 96.8 per cent recovery with chromium(VI) already in solution, no dose-response relationship with complete extraction and 5.8 per cent recovery on the solid as a solubility artefact. The chromate passes into the oil phase and is withdrawn from the measurement.
Is chromium(III) an end state?
No. The soluble Chromium(III) compounds carry the sensitisation marking Sh in the German MAK and BAT list 2026. Which compounds are actually present after a treatment is determined nowhere.
Does cleaning help permanently?
Cleaning and decontamination work for the single operation and do not remove the material cause. Where the calcium bearing insulation stays on the chromium alloyed hot component, the chromate forms again at the next thermal load.
Combustion ashes
Why is substitution not possible with ashes?
With thermal insulation the formation can be prevented through the choice of material. With incineration residues that option does not exist, because the reaction partner is in the feed material and the feed material cannot be chosen.
How much calcium oxide is in household waste?
Calcium oxide is present in household waste at about 3 percent.
In which temperature window does calcium chromate form in the ash?
Calcium chromate forms between 500 and 850 °C where the oxygen partial pressure is sufficient, and above 850 °C it is reduced again.
When is the plant critical, during full load operation?
The critical phase is not full load operation but shutting down for maintenance. Ash residues remain on the grate and on chromium alloyed hot components, the temperature passes through the formation window from above, and opening the plant admits additional oxygen.
What happens during the internal overhaul?
Scaffolding is erected and grate components are walked on, surface treated, removed and replaced.
Does direct desulphurisation make the situation worse?
Where the plant uses direct desulphurisation, calcium bearing sorbent is also injected into the furnace at about 900 °C or into the boiler passes at about 350 °C. For this field only the protective regime remains.
Fields of application and order of magnitude
Where does this occur?
On insulated chromium alloyed hot components: combined heat and power plants and cogeneration, gas and steam turbines, power plants, industrial furnaces and boilers, waste incineration and biomass plants, chemical plants and refineries, ships from the two-stroke engine to the yacht, locomotives and rail vehicles, road vehicles and construction machines and mobile working machines.
Which components are affected?
Exhaust lines, exhaust manifolds, turbochargers, indicator valves, boiler passes and grate components, each with their insulation and its cladding.
How many installations are there?
Public manufacturer statements, fleet statistics and plant registers give, among others, 190,000 engines of a single series in the field, more than 100,000 engines sold from two further series, an active installed base of 44 GW at one gas engine manufacturer, more than 7,000 gas turbines from one manufacturer with a stated share of about half of the global base, around 7,300 coal fired units, around 109,000 ships, 6.16 million trucks above 3.5 tonnes in the European Union and 2,539 waste incineration plants in Europe, the United States, China and Japan.
How many occasions does that give per year?
With the maintenance and overhaul intervals applied here, this gives about 58,000 to 61,000 occasions per year in the industrial segments alone, plus millions of workshop visits whose relevant share is not quantified.
Is that a projection of the findings?
No. The expert analysis contains a projection of the occasions, not of the findings.
The duties in Germany and the European Union
What does the German TRGS 561 say about it?
It covers the case under 5.9.3 and applies the protective regime as soon as a Chromium(VI) burden cannot be excluded.
Must I check before starting work whether hazardous substances are released?
The German Hazardous Substances Ordinance requires in Section 6 paragraphs 2a to 2c the examination of whether hazardous substances can be released from the plant during the work, the use of external expertise where the employer lacks the knowledge and, where a technical investigation is needed, that investigation as a precondition for the work.
May I start before the risk assessment is in place?
No. Section 7 paragraph 1 prohibits starting the work before the risk assessment and the protective measures are in place.
In which order must the measures be taken?
Section 7 paragraph 3 requires substitution as a matter of priority; paragraph 4 orders the further steps: the design of the processes and low emission forms of use according to the state of the art, then collective technical measures at the source of the hazard, and only then individual measures including personal protective equipment. Section 7 paragraph 5 sentence 2 provides that burdensome personal protective equipment may not be a permanent measure.
What does European law require?
Directive 2004/37/EC has required for more than twenty years that the nature, degree and duration of exposure be determined and that the further routes of exposure, including absorption through the skin, be taken into account. It applies in all Member States.
How high is the European limit value?
Since 17 January 2025 a binding limit value of 0.005 mg/m³ as a time weighted average over eight hours has been in force for chromium(VI) compounds. The transitional column names 0.010 mg/m³ until 17 January 2025 and 0.025 mg/m³ until the same date for welding and plasma cutting work and similar fume generating processes.
Does the Directive apply only to substances that are bought?
No. Article 2 letter a defines the carcinogen through the classification criteria of Annex I of the CLP Regulation and contains no restriction to substances that are bought, placed on the market or deliberately used. Calcium chromate and sodium chromate are classified in harmonised form as carcinogenic category 1B in Annex VI Part 3.
The duties in France, the Netherlands, Switzerland, the United Kingdom and the United States
How do the limit values compare?
| Legal system | Limit value as an eight hour average |
|---|---|
| European Union | 0.005 mg/m³ |
| France | 0.001 mg/m³ |
| Netherlands | 0.001 mg/m³ as chromium |
| United Kingdom | 0.01 mg/m³, process generated 0.025 mg/m³ |
| United States | 5 µg/m³ with an action level of 2.5 µg/m³ |
| Switzerland | no MAK value, because no carcinogenic threshold is known |
What is different in France?
Article R. 4412-61 of the Code du travail requires for any activity liable to present a risk of exposure the assessment of the nature, the degree and the duration of the exposure, including absorption through the skin. Article R. 4412-66 requires substitution as far as this is technically possible and the recording of the result in the document unique. Compliance is verified at least once a year by an accredited body, and an exceedance stops the work at the workplaces concerned.
What do the Netherlands require?
Article 4.2 of the Arbeidsomstandighedenbesluit requires the assessment of the nature, the degree and the duration of the exposure as part of the risk inventory, even where such activities are not currently carried out. Article 4.17 requires the priority replacement of carcinogenic substances and processes as far as technically feasible.
What is the position in Switzerland without EU membership?
The duties follow from Article 82 paragraph 1 of the Federal Act on Accident Insurance, which requires the employer to take all measures that experience shows to be necessary, that the state of the art makes applicable and that are appropriate to the given circumstances, and from the Ordinance on the Prevention of Accidents and Occupational Diseases. In the place of a limit value stands the minimisation requirement under the STOP principle, with substitution in first place.
Is there a prohibition in the United Kingdom on starting work before the assessment?
Yes, not substance-specific but in Regulation 6(1) of the COSHH Regulations 2002: an employer shall not carry out work which is liable to expose any employees to any substance hazardous to health unless a suitable and sufficient assessment of the risk has been made and the resulting steps have been implemented. For carcinogens the COSHH Regulations require at the same time reduction to the lowest level reasonably practicable.
What do the United States require?
29 CFR 1926.1126(d) requires every covered employer to determine the eight hour time weighted average exposure for each employee exposed. Added to this are protective clothing, change and washing facilities and a ban on blowing down with compressed air. The procurement clause DFARS 252.223-7008 prohibits the delivery of items containing more than 0.1 percent Chromium(VI) by weight in any homogeneous material, and likewise the delivery of items where Chromium(VI) would have to be removed or reapplied in later sustainment phases.
Is there anywhere a threshold below which the protective regime ceases to apply?
None of these systems recognises a threshold below which the protective regime would cease to apply, and in all of them the sanction attaches to the duty that was not performed rather than to the harm that occurred.