chrom i ting

7/31/2019 Chrom i Ting

1/11

Chrom tingThe Thick Layer Passivation for Zinc and Zinc Alloys

Free of Hexavalent Chromium

SurTec GmbH, SurTec-Strae 2, D-64673 Zwingenberg

1

SurTec GmbH April 2001

SurTec

w Corrosion and Corrosion Protection

w Cathodic Corrosion Protection of Steel

w Properties of Conventional Chromates

w Application, Properties and Corrosion Protection of Chrom ting

w Outlook

Topics 2



2/11

w steel is dissolved in salt and air containing water while oxygen is reduced and hydrogen

is evolved, and forms iron oxide/hydroxide which precipitates as a voluminous corrosion

product on the surface = Red Rust

w iron oxide layers are not protecting against further corrosion, in contrary to chromium

oxide on metallic chromium or aluminium oxide on aluminium

water

Fe2+

Fe(III) hydroxide and oxide

iron

q corrosion protection is provided:

barrier layers oil, conversion layers, lacquer, combination, metallic layersredox buffer less noble layers, conversions layers with different oxidation levels,

organic corrosion protectors

duplex layers combination of both

Corrosion and Corrosion Protection of Steel 3


w this is the typical method to protect steel and other iron material, because in the

electrochemical series of elements, iron is more positive than zinc, and this means

w zinc is less noble than iron and represents the anode in the galvanic element - iron is the

cathode

w thus, iron will be - as the more noble metal - protected cathodically, until the zinc is

corroded completely

q mechanism:

iron

zinc

chromatingwaterFe2+ Zn2+

Zn / Zn2+ : Zn Zn2+ + 2 e-

Fe / Fe2+ : Fe2+ + 2 e- Fe

Cathodic Corrosion Protection of Steel 4



3/11

w basic task is the protection of the base metal against rust and consequently also against

hydrogen induced embrittlement

w side purposes are a decorative function (brightness, colour) as well as tribological

properties

w higher layer thickness and more even metal distribution are extending the cathodic

corrosion protection

w zinc alloys (Co, Fe, Ni, Sn) are more noble than pure zinc and extend also its cathodic

corrosion protection

w in addition to the cathodic protection, the zinc layer is passivated by a chromate layer,

which is improving strongly the corrosion protection (by its barrier effect)

w the colour is defined by the chromating: possible tints are transparent-blue and

transparent-green (trivalent) as well as yellow, black and olive green (hexavalent)



metal distribution



conventional cyanide-free alkaline zinc process

new cyanide-free alkaline zinc process

9,9 13,8 14,2 14,2 15,6 15,1 14,1 m

15,1 15,5 17,5 12,3 11,8 17,0 20,9 m

conventional cyanide-free alkaline zinc process

new cyanide-free alkaline zinc process

9,9 9,5 9,6 9,3 9,3 9,6 9,6 m

11,9 7,0 8,6 7,1 9,6 11,1 m6,6


4/11

zinc

mechanism: cathodic protection

parameters: thickness, metal distribution, trace metals (e.g. alloys)

passivation

mechanism: barrier effect, specific effect of chromium(VI)

parameters:

type chromium(VI) in thechromate layer in mg/m

layer thickness in nm corrosion protection inhours salt spray test until

first white corrosion

blue (trivalent) 0 25-80 20-40

yellow 80-220 250-500 200-300olive-green 300-400 1000-1500 400-500

black 80-400 250-1000 150-300

2

Corrosion and Corrosion Protection of Steel 7


edge of a broken steel sheet, bright zinc plated andhexavalent yellow passivated, at a magnification of 40,000x

edge of a broken steel sheet, bright zinc plated and trivalentblue passivated, at a magnification of 40,000x

q if chromium(VI) had a specific corrosion protecting effect, we shall always expect a

disadvantage with trivalent chromates with regard to its anticorrosive properties

Properties of Conventional Chromates 8



5/11

q if the corrosion protection of a chromate layer was based on it barrier effect only, then

trivalent passivations with the same layer thickness should provide the same corrosion

protection as hexavalent passivations

edge of a broken steel sheet, bright zinc plated and trivalentchromited, at a magnification of 40,000x

Blauchromatierung

dreiwertig

Gelbchromatierung

sechswertig

Chromitierung

dreiwertig

0

50

100

150

200

250

300

350

400

450

Trommel Gestell Trommel Gestell Trommel Gestell

StundenimS

alzsprhschrank(DIN

50021)

bis 10 % WR

bis 5 % WR

bis Erstangriff

Corrosion Protection Comparison in Comparison

Properties of Chrom ting 9


barrel rack barrel rack barrel rack

Blue Chromate Yellow Chromate Chromiting

trivalent hexavalent trivalent

Corrosion Protection in Comparison

hoursin

saltspray(DIN5

0021

SS)

to 10 % WC

to 5 % WC

to first attack

trivalent Chromiting hexavalent yellow chromate

make-up: 12.5 Vol% 1 Vol%

= 10 g/l Cr(III) = 2 g/l Cr(VI)

pH-value: 1.8-2.2 1.6-1.8

temperature: 60 C room temperaturecontact time: 60 s 20 s

heating: necessary in winter season

exhaust: necessary recommended

rinsing: 3 steps 2 steps

activation: recommended

agitation: rack agitation/barrel rotation and/or air agitation

tank material: (insulated) plastic tanks or steel tanks with plastic inliner

Application: Chrom ting in Comparison to Yellow Chromate 10



6/11

The Green Alternative 11


0,0 0,2 0,4 0,6 0,8 1,0

100

80

60

40

20

0

%

sputter depth in m

chromium

zinc

Yellow Chromate

0,0 0,2 0,4 0,6 0,8 1,0

100

80

60

40

20

0

Chromiting

%

sputter depth in mchromium

zinc

Sputter Profiles in Comparison 12



7/11

Layer Thickness in nm

ellipsometry, glow discharge spectroscopy (GDOES) and scanning electron microscopy

immersiontime in s

temperaturein C

pH-value scanning electronmicroscopy

glow dischargespectroscopy

ellipsometry

Chromiting 60 60 2.0 300 312 353

120 60 2.0 400 440

60 60 2.5 195 230

90 60 2.5 295 300

120 60 2.5 435 400

60 100 2.0 360 580

in comparison:

Blue Chromate 60 60 100

Yellow Chromate 300 440 395

Chrom ting Layer Thickness Measured with Different Methods 13


depending on the immersion time

300

250

200

150

100

50

0

Chrom

350300250200150100500Chromitierung

ChromiumC

ontentinmg

/sqm

Immersion Time in s

Blue Passivation

Chromi

ting

Chromium Content in the Chrom ting Layer 14



8/11

depending on the treatment temperature

A B C D

1

23

4

5

6

7

8

9

1 0

1 1

1 2

1 3

1 4

1 5

1 6

1 7

Temperature Chrome Temperature Corrosion Protecti

25 50,8 25 18030 63 35 240

35 59,3 45 270

40 60,7 55 310

45 63,2 65 320

50 67,1 75 330

55 71,1 80 400

60 80,3 90 500

65 83,8 100 700

70 87,8

75 92,8

80 96,3

90 123,7

100 181,3

200

150

100

50

0

1009080706050403020

800

700

600

500

400

300

200

100

0

1009080706050403020

ChromiumC

ontentinmg/sqm

Treatment Temperature in C Treatment Temperature in C

Cor

rosionProtectioninhsaltsprayDIN50021SS

Chromium Content and Corrosion Protection 15


w old members Collini, Holder, Vollmerhaus

new: Holzapfel, Huster, Kronenberger, Zeschky

w application of Chromiting, Black Chromiting and possible further developments

w exchange of experience (trouble shooting)

w commitment to keep narrow parameter ranges for the coating

w development and guarantee of common standards

(measuring procedures, setting and guaranteeing process safety, reproducibility)

q task: to provide capacity in a sufficient volume and in constantly high quality

German "Qualittsfrderkreis Chromitierung" (just founded) 16



9/11

1. Chrom ting on Zinc

The natural colour is transparent and significantly green iridescent. This colour is no body colour

caused by pigments, but a layer thickness related interference colour. A significantly green layer has a

thickness of 100-700 nm.

This colour will disappear completely after an organic sealing with SurTec 552 or 555 (differentrefraction index than the passivation itself).

2. Chrom ting on Zinc/Iron or Zinc/Nickel

The colour of Chromiting on zinc/iron is stronger iridescent and slightly darker. Also here it is

disappearing to a great extent by an organic sealer, resulting in a colour similar to that of titanium.

On zinc/nickel, a thinner Chromiting layer appears dark blue - iridescent; thicker layers are looking

yellowish-green. After sealing, a uniform grey shade is formed.

2. Chrom ting on Other Surfaces

On zinc/cobalt, the colour is similar to zinc/iron, but the layer composition of zinc/cobalt is lessuniform and so is its colour. On hot dip zinc, it is opaque greyish green, on zinc dye cast often

spotted (copper!).

Colour Effects 17


What About Black Chromitings?

x black layers on zinc/iron are possible with a variation of Chromiting; its further

optimisation is a current project

x black layers on zinc/cobalt are possible with the same solution (but less uniform)

x for zinc/nickel there is no Black Chromiting up to now

Dyeability

x high-coloured dyes can be used to tint the layer; blue, yellow, "brass" and pinkish-red

are already tested

x black dyes (as for aluminium) are to weak in colour to result in an opaque black colour

Outlook: Other Colours 18



10/11

w minimal traces (between 1-3 mg/m2) of chromium(VI) are always present in high

amounts of chromium(III), due to the chemical equilibrium

w the analysis of chromium(VI) traces in chromium(III) bulk is not trivial

q a theoretical replacement for chromium(VI) in passivations must fulfil a series of criteria:

Chemical Properties

good water solubility in acidic pH (a conversion layer always requires a prior etching)

the formation of oxides, insoluble in water, acids and lyes

General

availability (mass production)

recyclability low raw material costs

known toxicology of all possible oxidation levels

Outlook: Completely Chromium-Free Layers? 19


the solubility of element oxides is coded in greys.light grey = insoluble in water; grey = insoluble in acids or lyes; dark grey = insoluble both in acids and lyes

Group

Period

* Lanthanides** Actinides

Outlook: Completely Chromium-Free Layers? 20



11/11

Conclusion

q Chromium(III) is the best alternative to Chromium(VI), other - more appropriate -

elements are not in sight

q with trivalent thick layer passivations (Chromiting), the requirements of the EU End of

Life Vehicles Directive are easily met

(2 g chromium(VI) per car are allowing 600 to 2000 m2 passivated surface)

q in collaboration with the VDA, chemical suppliers are developing an analytical method

for the detection of chromium(VI) in passivations

Outlook 21


chrom i ting

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