Passivation With Trivalent Chromium

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Middle East Technical University Metallurgical and Materials Engineering Department ALTERNATIVE PASSIVATION METHODS for HDG Barış Yiğit Alpay

description

alternative method for toxic hexavalent chromium. this document is about the passivation after hot dip galvanizing mainly.

Transcript of Passivation With Trivalent Chromium

Page 1: Passivation With Trivalent Chromium

Middle East Technical UniversityMetallurgical and Materials Engineering Department

ALTERNATIVE PASSIVATION METHODS for HDG

Barış Yiğit Alpay

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• Zinc is an electrochemically high reactive metal and

its corrosion rate may be high particularly under

outdoor conditions. [1]

• For this reason, a post-treatment is necessary in

order to increase the lifetime of zinc coatings.

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Previously

• Passivation with Cr(VI)

• Passivation with Silane

• Passivation with a Chrome free solution

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Outline

• Passivation with Cr(III)

o Ingredients of the bath.

o Information about each ingredient.

o Composition of the bath.

o Tests, results, objections.

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Aim of Cr(III) Passivation

• A Cr(VI) free passivate; which performs as well as, and in some aspects better than Cr(VI) passivateswhich contains:

o No Cr(VI)

o No Peroxide

o No Nitrates

• Non toxic.

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Cr(III) Passivation Solution Contains [2]

• Water

• Phospate Ions

• Cr(III) Ions

• Anions of at least one complex fluoride of Ti,Si,Zr

• Free fluoride ions

• Organic acid inhibitor

• A pH adjusting component

• Organic substance that reduce Cr(VI) to Cr(III)

• Inorganic metal compound(s)

• Additive(s) selected from, sequestrant, wetting agent, defoamer

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Phosphate Ions

• Variety of sources available, most common is

phosphoric acid and its salts for economical

reasons.

• They increase bath stability.

• They may lower the corrosion resistance.

• Concentration of phosphate ions is at least 2g/L, at

most 400g/L of total composition. 17-34g/L is

preferred.

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Cr(III) Ions

• Source may be salts of Cr(III) cations.

• Another source is Cr(VI), for economical reasons.

However;

o Cr(VI) to Cr(III) conversion must be achieved.

o Thus, a reducing agent must be added, i.e. tannic acid,

alcohol (sorbitol), sucrose…

• Concentration of Cr(III) is at least 1g/L, at most

75g/L of total composition. 17-20g/L is preferred.

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Ratio of Phosphate Ions to Cr(III) Ions

• It is found that reducing the ratio and adding

complex fluoride anions results in greatly improved

corrosion resistance.

• Ratio must be at least 0.1:1.0, at most 7.5:1.0,

0.9:1.0 to 1.5:1.0 is preferred.

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Complex Fluoride Anions of Ti, Si, Zr…

• Can be added as acids or salts.

• Can be formed by appropriate oxides in presence of

HF.

• It must be water soluble.

• Formula is HpTqFrOs. H2SiF6 is preferred.

• Concentration of anions is at least 0.5g/L, at most

60g/L for economical reasons. 13-23g/L is

preferred.

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Free Fluoride Ions

• Can be supplied from any possible source. Hydrofluoric acid is preferred for economical reasons.

• Concentration is at least 0.1g/L, at most 25g/L. 3.3-3.5 g/L is preferred.

• Higher the composition, higher the stability of the solution. But too high concentration can lead to a too vigorous attack on the metal. This leads to a less corrosion resistant surface.

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Organic Acid Inhibitor

• They include a quaternary ammonium compoundlike N-alkyl, N-cycloalkyl, etc…1-benzylquinoliniumhalide is preferred.

• They provide reduced dissolution of metal into thebath, but do not stop the etching of the surfacewhich is necessary to deposit the passivatingcoating.

• Concentration is at least 0.001g/L, at most 2g/L. 0.10-0.15 g/L is preferred.

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A pH Adjusting Component

• pH of the solution can be adjusted by addition of an

acid. Phosphoric acid may be preferred.

• Acids are added to lower the pH and optimize the

effectiveness of the solution.

• pH of the solution should be from 0.5 to 5, most

preferably 1.5 to 2.5.

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Organic Substance

• It is needed if there is initial Cr(VI) in the solution.

• It reduces Cr(VI) to Cr(III).

• After reducing, they may leave the system as gas,

or remain in the system.

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Inorganic Metal Compound

• They are oxides and carbonates of Co, Si, Ni, Zn.

50 ppm to 1500 ppm of Zn in the bath is preferred.

• They may improve corrosion resistance of

passivated surface.

• They may initiate deposition of the coating onto the

surface.

• They are soluble in bath, thus they are incorporated

into the coating.

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Sequestrant, Wetting agent, Defoamer

• To facilitate the coating process, one or more of

them may be used.

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Composition of a Bath

• 10-30 wt % Phosphate Ions

• 2-8 wt % Cr(III)

• 1-20 wt % Complex Fluoride

• 0.01-0.09 wt % Organic Acid Inhibitor

• 0-2 wt % Fluoride Ions

• Sufficient amount of acid to set pH to 0.2-5

• 0.001-0.05 wt % Zn (optional)…

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Neutral Salt Spray Tests

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Objection:Tests Done by Tom Rochester [3]

• 1,5-diphenyl carbohydrazide is used.

• It forms a complex with Cr(VI) at low pH. It has red-

violet color.

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Results of the Tests

• A Cr(III) passivated article is placed in an operating

ASTM B-117 Salt Spray Chamber with a drop of

1,5-diphenyl carbohydrazide test solution on the

surface of the article.

• After 24 hours, the drop has turned reddish-violet,

indicating the presence of Cr(VI). Meaning Cr(III) is

oxidized to Cr(VI) in the chamber.

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How Does Cr(III) Protects the Steel? [4]

• There are 4 ways to protect a metal substrate from

corrosion:

1. Impressed electrical current

2. Barrier protection

3. Sacrificial protection

4. Inhibitive protection

• Cr(III) passivations protect Zn substrates by

generating Cr(VI) during the corrosion process.

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EU Specifications

• According to EU, the coating cannot contain more

than 0.1% hexavalent chromium.

• But, even tough there is no Cr(VI) initially, it may

form due to the corrosive atmosphere later on.

• EU has not specified a way of measuring the mass

of the coating, and has not approved a test for the

concentration of Cr(VI).

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Function of Cr(III) in the Waste Treatment [4]

• Sulphur dioxide reduces Cr(VI)to Cr(III) at pH less

than 2.

• 2H2CrO4 + 3H2O + 3SO2 Cr2(SO4)3 + 5H20

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References

• [1]C.R.Tomachuk et. al. “Corrosion resistance of Cr(III) conversion treatments applied on electrogalvanisedsteel and subjected to chloride containing media”, 2009

• [2]McCormick et. al. “Trichrome Passivates for TreatingGalvanized Steel”, US2009/0266450 A1, 2009

• [3]Tom Rochester, Zachary Kennedy, “UnexpectedResults from Corrosion Testing of Trivalent Passivates”, 2007

• [4]Tom Rochester, “Behavior of Trivalent Passivates inAccelerated Corrosion Tests”, 2009