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Since some chrome plating production processes generate hexavalent_chromium compounds, which are reportedly highly toxic/carcinogenic, I wonder:

Do the hexavalent chromium substances only occur (temporarily) during the production process (i.e. the chrome plating) or will they remain with the product?

My naïve impression has been that after the chrome plating process is over, the product will have an additionally a more or less thick layer of native Cr. Is this correct?

Should I expect impurities or residuals of toxic forms (e.g. hexvalent chromium) to be with the chrome plated this is possible do primitive test exist to detect product?

If it is possible that the chrome plated object could have worrisome concentrations/amounts of Cr(IV), is there a primitive test to detect this?

Update Some internet resources, such as tests/detection procedures for Cr(IV) described here, mention

The screening method can be used to determine chromium(VI) in passivation layers on metal pieces.

Also test kits for Cr(IV) sold online mention in their description to be

test[s] to detect leachable Cr (VI) ions on virtually any solid surface.

both giving reason to thing that the Cr (VI) might remain in worrisome amounts after the chrome plating. True.

update 2

one of the answers mentioned that "chrome plating", being understood as a general term, does not necessarily imply which material is being plated. If permissible I would like to add that I am most interested in those risks of the base material being copper.

update3

The most upvoted answer has stated little risk if chrome plating procedure is carried out well.

As new information the CHROME PLATINGANDCHROMIC ACID ANODIZING SURVEY strongly suggests that the term chrome plating, can be divided regarding the categories a) Tank process (Hard plating; decorative; chromic acid anodizing) and b) *Tank constituent(s) (Hexavalent chrome; trivalent chrome).

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This information to raises the aspect, if the outer passivated layer should be expected to have only Cr(III), irrespective of the used tank constituents.

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  • $\begingroup$ Many production processes involve toxic intermediates that don't make the final product. PVC doesn't contain vinyl chloride and polystyrene doesn't contain benzene but these are big in the manufacturing routes for the polymers. $\endgroup$
    – matt_black
    Commented Feb 7, 2019 at 16:29

3 Answers 3

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Multiple questions should be broken in multiple items, but anyway,

If your plating process is well done, and everything is thoroughly cleaned after, the level of Cr(III) or Cr(VI) remaining should be low enough to be inconsequential to health and the environment. You're correct stating that regardin chrome, thre will be only a passivated layer of metallic chromium and not the trivalent or hexavalent.

Please do remember that the main reason that chrome plating makes things resistant to corrosion is that as soon as chromium is exposed to air, it forms a very thin layer of chromium oxide attached to the surface of the metal, preventing further corrosion. So, in fact, there is some Cr(III) on a finished piece of chrome plated material.

We aren't considering the environment were the finished part will be exposed/used. If aggressive enough conditions are present, the chromium layer will be corroded away and will enter the environment as Cr(III)/Cr(VI).

Regarding Cr(VI) detection, there is a "simple" method involving a solution of diphenylcarbazide described at https://www2.mst.dk/udgiv/publications/2009/978-87-7052-987-7/html/kap04_eng.htm

However, one can easily buy test kits for Cr(VI) on Amazon, ready for use, starting at about (February 2019) USD 110.

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  • $\begingroup$ Thank you. Since you mention the concept "further oxidation is prevented by a tiny outer fine of chrome (III) oxide", this is also true for many so called stainless steel alloys, correct? I mean with regard to Cr(III)-oxide, that is the outer layer of most kitchen pots, right? $\endgroup$
    – humanityANDpeace
    Commented Feb 7, 2019 at 16:42
  • $\begingroup$ In the link you posted (which describes Cr(IV) detection), it lists quote: "The screening method can be used to determine chromium(VI) in passivation layers on metal pieces." which would suggest, that passivation layers (what I understand to be the Crome plating, correct) are indeed something at risk to contain "leachable Cr(IV)". $\endgroup$
    – humanityANDpeace
    Commented Feb 7, 2019 at 17:17
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    $\begingroup$ Might be worth noting that in the past there have been plenty of occurrences of companies mishandling Cr(VI), leading to permanent disabilities and illness like leukaemia. In other words, it's quite easy to screw it up. $\endgroup$
    – Mast
    Commented Feb 8, 2019 at 14:34
  • $\begingroup$ @Mast Does this extend beyond mishandling in Chrome plating? For instance most stainless steel (especially US 304, US 316 food grade) contain considerable Cr amounts, could those be misshanded or are alloys impossible to contain CrIV in considerable amounts? $\endgroup$
    – humanityANDpeace
    Commented Feb 9, 2019 at 8:38
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    $\begingroup$ If chrome-holding paint is not considered plating, then yes, it goes beyond that. With regards to stainless steel, I honestly don't know. I wouldn't exclude the risk without further study. $\endgroup$
    – Mast
    Commented Feb 9, 2019 at 12:48
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If we assume that the plating process was done correctly onto tin then according to a XPS study on a chromate conversion layer on tin plate (J. Sun et. al., SURFACE AND INTERFACE ANALYSIS, 2009, volume 41, issue 6, pages 449-452) the conversion layer is a combination of mainly chromium(III) oxide, chromium(III) hydroxide, tin and tin oxides.

However in a study on a chromate conversion layer on zinc, it was found that much of the chromium was still present in the form of chromium(VI). The paper in question was X. Zhang et. al., SURFACE & COATINGS TECHNOLOGY, 2005, volume 197, issue 2-3, page 168-176.

Now this suggests that chromium(VI) is present in the films, I know that the chromium in soil can change from one oxidation state to another. If the soil is alkaline and MnO2 is present then it is possible for chromium(III) to be converted into chromium(VI). I think that the question suffers from the problem that we do not know what the metal is under the chromate conversion coating , also we do not know what environment the object is kept in. I think we know too little to be able to give a perfect answer.

In a study on chromate conversion layers on aluminium, it was found that soaking a film in sodium chloride caused chromate to be leached from the layer. This was published by J. Wan, Physica B, 1995, volume 208, issue 1-4, pages 511-512.

So I think it is possible for chromate to be released.

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  • $\begingroup$ I thank you for bringing the aspect of the base material. I did not specify because I thought that different processes (i.e. different chemical composition of solutions used in the plating process) have a higher impact on how much Cr(IV) will be contained in the outcome. However if there was something specific to Cu as the base material being chrome plated, then it would be great to know about that. $\endgroup$
    – humanityANDpeace
    Commented Feb 7, 2019 at 21:22
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The answer given above by Flyingfenix is on the spot. However, I felt reader may need to know the possible interference of the test mentioned to check $\ce{Cr(VI)}$ in finished products. This method is called Method 7196 by EPA (USA). They have published that Method 7196 may be used to analyze samples containing from $0.5$ to $\pu{50 mg}$ of $\ce{Cr(VI)}$ per liter ($\pu{0.5-50 ppm}$ levels). In Method 7196A, it was mentioned that:

3.1 The chromium reaction with diphenylcarbazide is usually free from interferences. However, certain substances may interfere if the chromium concentration is relatively low. Hexavalent molybdenum and mercury salts also react to form color with the reagent; however, the red-violet intensities produced are much lower than those for chromium at the specified pH. Concentrations of up to $\pu{200 mg/L}$ of molybdenum and mercury can be tolerated. Vanadium interferes strongly, but concentrations up to $10$ times that of chromium will not cause trouble.

3.2 Iron in concentrations greater than $\pu{1 mg/L}$ may produce a yellow color, but the ferric iron color is not strong and difficulty is not normally encountered if the absorbance is measured photometrically at the appropriate wavelength.

The other issue I have with the test is its use of ethanol as a solvent to prepare diphenylcarbazide solution. However, major theory of the test is $\ce{Cr(VI)}$ would oxidize diphenylcarbazide to diphenylcarbazone, which would produce a red-violet color. But, it's a known fact that ethanol itself would get oxidized by $\ce{Cr(VI)}$. For example, $\ce{K2Cr2O7}$ would oxidize ethanol at $\pu{25 ^oC}$ with rate $K_{\mathrm{obs}} = \pu{0.193 min^{-1}}$ (pseudo-first-order kinetics; Ref.1). Don't you think it would (negatively) interfere as well? Other than that, this is a good test for quick determination of the presence of $\ce{Cr(VI)}$.

Reference:

  1. M. Hassan, A. N. Al-Hakimi, M. D. Alahmadi, “Kinetics of Oxidation of Aliphatic Alcohols by Potassium Dichromate in Aqueous and Micellar Media,” S. Afr. J. Chem. 2011, 64, 237–240 (https://www.ajol.info/index.php/sajc/article/view/123748).
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  • $\begingroup$ In your answer you mention that "[t]he answer given above by Flyingfenix is on the spot.", which suggested that "If your plating process is well done, and everything is thoroughly cleaned after, the level of Cr(III) or Cr(VI) remaining should be low enough to be inconsequential to health and the environment.". I would be interested in how you understand this sentence, in particular what is meant by "cleaned after" part. $\endgroup$
    – humanityANDpeace
    Commented Feb 7, 2019 at 21:18
  • $\begingroup$ Your concern should be answered by @Flyingfenix, because you are directing to his comments. My article is purely based on test for the presence of $\ce{Cr(VI)}$. $\endgroup$
    – Mathew Mahindaratne
    Commented Feb 7, 2019 at 23:37

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