Passivation Stages New Cell Stored cell Passivation Layer Depassivation Process.
Passivation With Trivalent Chromium
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Transcript of 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