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Chromate expertise

Chromium(VI) compounds

Chromium(VI) and its compounds (chromates) are carcinogenic hazardous substances, some of them also mutagenic and toxic to reproduction, and all of them harmful to the environment.

Classification

Hazard classes

Chromium(VI) as chromium trioxide (CrO3)

EC No 606-053-1 · CAS 18540-29-9

  • H317 may cause an allergic skin reaction
  • H350 carcinogenic 1B
  • H400 very toxic to aquatic life
  • H410 very toxic to aquatic life with long-lasting effects

Source: ECHA, C&L Inventory

Calcium chromate (CaCrO4)

EC No 237-366-8 · CAS 13765-19-0

  • H302 harmful if swallowed
  • H350 carcinogenic 1B
  • H400 very toxic to aquatic life
  • H410 very toxic to aquatic life with long-lasting effects

Source: ECHA, C&L Inventory

Sodium chromate (Na2CrO4)

EC No 231-889-5 · CAS 7775-11-3

  • H330 fatal if inhaled
  • H350 carcinogenic 1B
  • H340 mutagenic 1B
  • H360FD repro 1B, may damage fertility, may damage the unborn child
  • H400 very toxic to aquatic life
  • H410 very toxic to aquatic life with long-lasting effects

Source: ECHA, C&L Inventory

Chromium(VI) and its compounds therefore all belong to the CMR substances (carcinogenic, mutagenic and/or toxic to reproduction), and above all they are uniformly classified as carcinogenic 1B.

Formation

What are chromates?

Calcium chromate on the textile insulation of a CHP engine

Chromates are salts of chromic acid (H2CrO4). The chromate anion CrO42− is yellow and has a tetrahedral structure.

The chromates described here, such as calcium or sodium chromate, form thermochemically from chromium(III) oxide (Cr2O3), an alkali metal oxide (sodium oxide) or an alkaline earth metal oxide (calcium oxide) at temperatures between about 250 °C and about 800 °C in the presence of oxygen.

The higher the chromium(III) content, the higher the alkali or alkaline earth metal content and the higher the temperature, the higher the rate of oxidation of chromium(III) to chromium(VI).

When insulation materials containing alkali or alkaline earth metals come into contact with chromium-containing hot parts, calcium oxide or sodium oxide is released, in particular through vibration and thermal stress (abrasion, wear), and meets the chromium oxides of the stainless steel alloys at high operating temperatures. Over time calcium or sodium chromate forms, which settles as a fine yellow powder between the inner surface of the insulation and the hot part, or is carried around the plant by thermal currents.

For a long time the yellow dusts were mistaken for sulphur deposits. Calcium chromate also forms when calcium-containing assembly pastes or sealing materials are used on chromium-containing bolts, nuts and flange connections.

Analysis

The method decides the finding

The determination of chromium(VI) is a speciation analysis. An oxidation state is measured, and it can change during sample preparation. For thermal insulation the matrix itself is the reaction partner: rich in calcium, alkaline and, when used, interspersed with soot, oil and corrosion products. No standardised method exists for this matrix; according to the laboratory involved, transferred methods give results 20 to 30 per cent too low in mild cases and values close to zero for samples high in calcium.

Method

The SEEF method

The Dutch laboratory SEEF B.V. has developed its own method for the insulation matrix. It stabilises chromium(VI) throughout the analysis and, according to the laboratory, achieves recoveries structurally above 80 per cent on spiked insulation samples.

In a matrix with 16 per cent zinc, recovery was 0.2 per cent under NEN 5617, 6.5 per cent with another accredited method and 35.3 per cent with the SEEF B.V. method.

seefbv.com

Quick test

Why quick tests fail

Colorimetric quick tests are screening tools. A positive result is a reliable warning. A negative result does not show that there is no chromium(VI) contamination.

The French field study by Passeron et al. (2025) compared the quick tests with laboratory-analysed wipe samples at the same sampling points. On the turbocharger the quick test was negative while the laboratory found 869 ng of chromium(VI).

Regulatory analysis: quick test and neutralisation

Wipe test

The laboratory-analysed wipe sample

A reliable assessment of surface contamination requires representative wipe samples and laboratory determination of chromium(VI). They assess a different part of the hazard than activity-related air measurements.

A positive result proves presence. A negative result applies to the point tested, the depth tested and down to the detection limit of the method. After treatment with penetrating oil the sample gives no usable result; with WD-40 recovery was 5.8 per cent.

French field study, Passeron et al. (2025), maintenance of diesel and marine engines
Sampling pointLaboratory Cr(VI)Colorimetric quick test
Exhaust manifold127 ngat most faintly positive
Turbocharger869 ngnegative
Bolt at turbocharger outlet1,646 ngpositive

Absolute Cr(VI) quantities per sample. Sources: The Overlooked Ingredient, expert analysis V1.11, section 17 and section 19