27. Chromium(III): the assumed final state

All the methods treated in section 26 rest on the same assumption. That with the conversion of chromium(VI) into chromium(III) the dangerous part of the process is at an end. This section examines the assumption against the documents, the classifications and the chemistry of the process.

The line of argument in the manufacturer documents

Technical Instruction of , released on 31 May 2023, sets it out in full. Chromium(VI) is said to be “almost completely converted into the much less dangerous chromium of oxidation state 3” by the reduction solution. On the assessment of this state it says: “Cr-3, for example, is found in many foods. Daily intake levels in Europe are generally in a range where no health risk is expected. Whether Cr-3 is an essential trace element for humans at all has not been conclusively clarified. Since the possible concentration of Cr-3 after application of the reduction solution is far below the applicable limit values (occupational exposure limit values), a health hazard from Cr-3 can be ruled out.” Emphasised, there follows: “Unlike Cr-6, Cr-3 is not carcinogenic.” [25]

In its third sentence the paragraph concedes that essentiality has not been conclusively clarified, and in the fourth it rules out a health hazard. The reasoning relies on limit values without naming one. The question of essentiality was answered by the 2014 assessment of the European Food Safety Authority, which found no evidence of the essentiality of chromium(III) and considered the setting of an adequate intake to be inappropriate. [61]

What arises in the reduction

The MAK documentation for the chromium(VI) compounds of the inhalable fraction describes the process as a multi-stage reaction: “The reduction of chromium(VI) proceeds via chromium(V) and chromium(IV) to chromium(III), with radical oxygen and sulphur compounds being formed.” Which intermediate stage occurs depends on the reducing agent. In the reaction with ascorbate, two electrons are transferred in the first step and chromium(IV) is formed; with glutathione, one electron is transferred and chromium(V) is formed, which produces hydroxyl radicals in reactions analogous to the Fenton reaction. In all cases the end product is the stable oxidation state chromium(III). [128]

This account concerns the reduction inside the cell. Its transfer to the treatment of a component surface is an analogy and is identified as such here. What can be transferred is the fact that the conversion is not a single-step reaction to a defined end product.

For treatment on the plant there is in addition what lies on the surface. Depending on the instruction, the reducing agent is ascorbic acid, a mixture of acids or iron(II) sulphate, and in some cases citric acid is added. On the treated surface there are oils, greases, insulation material residues and assembly pastes. With organic acids and their oxidation products, chromium(III) forms soluble complexes. Chromium(III) oxide, which is what the product statements mean, forms thermally and not by spraying on and wiping off.

The unknown species

Which chromium(III) compounds are actually present after a treatment is stated in none of the documents considered. The work instruction of Vattenfall Generation NL does not name the reaction product, the product pages of Eco-Point and MATinspired speak of chromium-3 and non-toxic chromium-3 without stating a compound, the certificate of the testing body refers to the effectiveness of the conversion and not to its product, and the page of the German statutory accident insurance institution for the construction industry speaks in the plural of uncritical chromium(III) compounds without naming one. [115, 116, 117, 131, 132]

The analysis used likewise does not answer the question. The colorimetric rapid test indicates chromium(VI) or it does not. The laboratory methods determine chromium(VI) or total chromium. Speciation analysis of the chromium(III) compounds formed is not provided for in any of the methods and is not required in any of the documents.

Classification of the chromium(III) compounds

The MAK and BAT value list 2026 of the German Research Foundation lists the chromium(III) compounds without a MAK value, without a peak limitation, without a pregnancy risk group, without a skin absorption marking and without a category for carcinogenicity or germ cell mutagenicity. Under sensitisation the group carries the marking Sh for a skin sensitising effect, with the addition: “Does not apply to chromium(III) oxide and comparably poorly soluble chromium(III) compounds.” For comparison, the same list gives the chromium(VI) compounds of the inhalable fraction the skin absorption marking H, the sensitisation marking Sh, carcinogenicity category 1 and germ cell mutagenicity category 2. [102]

The Sh marking thus applies to the soluble chromium(III) compounds and expressly not to the oxide and comparably poorly soluble compounds. Assignment to the group without a marking presupposes knowledge of the species formed, which according to the preceding subsection is not determined. In the account given by the suppliers it is made without a finding.

For chromium(III) oxide itself a workplace limit value of 2 mg/m³ in the inhalable fraction applies in Germany, with excursion factor 1 and short-term value category I. For chromium(III) oxide there is no harmonised classification under the European CLP Regulation. [130]

The French agency Anses examined chromium(III) oxide as the evaluating Member State within the substance evaluation under the REACH Regulation and submitted its opinion on 28 July 2021. It considers a classification as a skin sensitiser of category 1 to be indicated and expressly regards this as relevant for other chromium(III) compounds as well. For the inhalation route a harmonised classification as STOT RE 2 with H373 is to be examined, because the lung damage in animal experiments is regarded as irreversible. For the environment it recommends the harmonised classification as Aquatic Acute 1 with H400 and Aquatic Chronic 1 with H410. On germ cell mutagenicity it concludes that, on an overall assessment, the criteria of the CLP Regulation are not met; on the nanoforms there are two studies giving cause for concern, whose conclusiveness is limited by the absence of data on the chromium(VI) content. On reproductive toxicity it regards the data as insufficient and recommends that a multi-generation study in accordance with OECD 443 and a prenatal developmental study in accordance with OECD 414 be required. [129]

The same opinion records that the European Classification and Labelling Inventory contains self-classifications for chromium(III) oxide including Carc. 1A, Muta. 1B and Repr. 2, which in part relate to chromium(VI) impurities, and it recommends that the enforcement authorities examine whether chromium(VI) above 0.1 per cent is present without a matching classification. [129]

Between the German and the French assessment there is an open point. The MAK and BAT value list excludes chromium(III) oxide from the Sh marking, while Anses considers a classification of the same substance as a skin sensitiser of category 1 to be indicated. Both assessments stand side by side. [102, 129]

The sensitising step is the chromium(III) step

The MAK documentation describes the course of skin sensitisation by chromates. Chromium(VI) compounds penetrate the horny layer and are reduced to chromium(III) in the skin, probably above all by sulphur-containing amino acids and glutathione. “Unlike the hexavalent compounds, these trivalent chromium compounds are able to bind to high molecular weight carrier molecules such as proteins and can act as haptens” and thus induce a sensitisation of the skin. Trivalent chromium is also involved in genotoxicity: chromium(III) formed in the cell nucleus forms adducts with the DNA, “predominantly ternary complexes of DNA, chromium(III) and small molecules such as ascorbate, glutathione, cysteine or histidine”, which can cause gene mutations, DNA double strand breaks, chromosome breaks and micronuclei. [128]

The precedent of leather

The same MAK documentation describes an application in which chromium(III) has long been used as a working material: “Tanning is indeed carried out with chromium(III) compounds, but where processing or storage methods are unsuitable small quantities of chromium(VI) compounds can form from them. Where the chromium(III) is formed in situ by reduction of chromium(VI) compounds, soluble chromium(VI) can likewise remain in the leather if the reduction is incomplete.” [128]

Both statements in this sentence concern the process dealt with here. Under unsuitable conditions chromium(VI) can form from chromium(III), and an incomplete in situ reduction leaves soluble chromium(VI) behind. For treatment on the hot component, completeness of the reduction is not assured according to the suppliers' own statements.

Mobility and the conclusion as to drain-free working

The methods described in section 26 work wet. A solution is sprayed on, wiping is done with cloths, and under the MATinspired instruction rinsing with water follows. This produces a liquid phase which, on the state of the suppliers' statements, can still contain chromium(VI), because complete conversion is not assured and, according to the certificate of the testing body, the release procedure is not reliable where the result is negative. To this are added the soluble chromium(III) complexes, whose composition is not determined. [132, 116]

On the disposal of this liquid phase the product pages and the questions and answers of MATinspired contain no statement, and the washing of the used cloths is expressly provided for there. For the same activity the operating instruction of the German statutory accident insurance institution for the construction industry requires the waste to be collected at the place where it arises in PE sacks or big bags and to be labelled. From both of these follows the requirement to carry out such work drain-free and to treat the liquid arising as hazardous substance waste as long as its chromium(VI) content has not been determined by a reliable method. [126, 132]

The return of the exposed chromium(III) to the hot component

In the alloy matrix the chromium is bound and accessible to a reaction only via the surface. After a reduction, chromium(III) compounds are present finely distributed and unbound on the surface and in the adhering material. This state is the starting state of chromate formation as documented in this analysis: the patent of 1939 describes the deliberate production of calcium chromate from chromium-containing material with lime and atmospheric oxygen, Mao et al. demonstrate the formation of calcium chromate from chromium(III) in the presence of calcium oxide in the temperature window from 500 to 900 °C, and Verbinnen et al. show the same dependence for alkali and alkaline earth compounds. [1, 9, 10]

If treated insulation material is refitted under the instruction of Vattenfall Generation NL and the plant is started up, all the conditions are met once again, and the reaction partner chromium is present in a more readily accessible form than before. No investigation of this case exists. The supplier's statement that re-formation has not been observed at room temperature does not cover it. [115, 132]

Delimitation

The lower toxicity of the chromium(III) compounds compared with the chromium(VI) compounds is undisputed and is not called into question here. Chromium(III) compounds carry no carcinogenicity category and no germ cell mutagenicity category. The statements that the reaction product is uncritical, harmless or non-toxic go beyond this. They make a statement about substances that are not determined, they leave the Sh marking of the soluble chromium(III) compounds unmentioned, they leave the official assessment by a Member State on skin sensitisation and on hazard to water unmentioned, and they presuppose a completeness of the conversion that no supplier assures.