passivation with trivalent chromium

Middle East Technical UniversityMetallurgical and Materials Engineering Department

ALTERNATIVE PASSIVATION METHODS for HDG

Barış Yiğit Alpay

• 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.

Previously

• Passivation with Cr(VI)

• Passivation with Silane

• Passivation with a Chrome free solution

Outline

• Passivation with Cr(III)

o Ingredients of the bath.

o Information about each ingredient.

o Composition of the bath.

o Tests, results, objections.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

Sequestrant, Wetting agent, Defoamer

• To facilitate the coating process, one or more of

them may be used.

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)…

Neutral Salt Spray Tests

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.

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.

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.

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).

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

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