© Alufinish 2010

Pre-treatment of Aluminium before Powder CoatingBy R.N.Singh

21 November 2018

We all are committed and it‘s

our responsibility to produce,

implement Environmentally

friendly products and

processes for our lives and

future generation

Environmentally friendly products and processes

S Degreasers which are borate free

S Passivation which are Chromium free

for our industry in corrosion protection for aluminium, steel and zinc substrates

Common Protection systems

Chromating and chrome free systems

✓ yellow chromating e.g. Alfipas 720/725

✓ green chromating e.g. Alfipas 731

✓ clear chromating ( rarely used ) e.g. Alfipas 721

✓ chrome free systems, containing metals e.g. Alfipas 745/746

✓ chrome free systems, free of metals e.g. Alfipas 740

✓ chrome free systems with no-rinse properties e.g. Alficoat 748, Alficoat 748/3

Yellow chromatingapplication: architectural purposes, aircraft and machine industry

properties: excellent corrosion protection on coated substrates; also for galvanized parts

coating weight: 0.6 – 1.2 g/m² on aluminium (Qualicoat)

colour of the coating layer: yellowish to orange

composition of the layer: aluminium / chrome(III) – oxides / chromates (CrVI)

Yellow chromating

Disadvantages:

☹ chromates / chromic acid is toxic; bath solution and coating layer contains Cr(VI)

☹ waste water treatment needs a decontamination procedure (chrome reduction)

☹ not for food packing

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Green chromating

application: architectural purposes, packing foils

properties: excellent corrosion protection on coated substrates; also for galvanized parts

coating weight : 0.6 – 1.5 g/m² on aluminium (Qualicoat)

colour of the coating layer : greenish

Composition of the layer: aluminium / chrome(III) – phosphates / oxidhydrates

Green chromating

Disadvantages:

☹ chromates / chromic acid is toxic, bath solution contains Cr(VI)

☹ waste water treatment needs a decontamination procedure (chrome reduction)

☹ unsuitable for iron and steel substrates

☹ insufficient protection against filiform corrosion

☹ bath solution must be partly renewed time by time

Quality Control

Chromating bath solution

• Determination of product concentration ( Cr6+ )• Determination of chromium (III) • pH – controlling ( yellow chromating )• Measuring fluoride content ( green chromating )

Metal surface

• optical evaluation• determination of the chromate coating weight ( g/m² )

to escape...Why chrome-free ?

chromic acid, dichromic acid, chromtrioxide and different salts are put on the SVHC – candidate-list by the ECHA

(Substances of Very High Concern)(European Chemicals Agency)

Chrome-free

Conversions

Nano

Working conditions and practical experienceOld technology Chromefree Process

Acidic etching and degreasing Acidic etching and degreasing

Rinse Rinse

Rinse DI - rinse

Yellow chromating Chromefree

Rinse

DI - rinse

Fogging (mist)Draining or air blasting

Drying

pretreatment

rinse

rinse

rinse

Chromate

rinse

DI - rinse

pretreatment

rinse

rinse

DI - rinse

Chromfree (Ti)

rinse

DI - rinse

pretreatment

rinse

rinse

DI - rinse

Chromfree (Ti) NR

( DI - mist )

Chromating Chrome-free + rinse Chrome -free no-rinse

Aluminium surface

ChromateChrome-free

ca. 500 nm

ca. 30 nm

ConversionLayersComparison

ca. 5000 nmPreanodised

human hair:

Ø 70000 nm

1 µm = 1000 nm

Can you see the

chrome-free layer ?

normally: not

but…

Process Method Image

Chromfreelayer

Quick test / colour drop

Photometric (mg Ti/m²) on place

Control in Lab Alufinish(X-ray-spectroscopy)

Measuring methods and detection‘ Quick test ’ for Chromefree presence

• By colour indicator solution• Dropped on the hot surface• Creates a pink to purple colour• In relation to the amount of titanium

without normal heavy coating

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Approvals and corrosion testing results

Test According to Positive results afterConstant climate condensation water test

DIN 50017 KK 1000 hours

Resistance to humid atmospheres containing SO2

EN ISO 3231, 0.2 l SO2 24 cycles

Acetic salt spray resistance ISO 9227 1000 hours

Machu test 48 hours

Excellent corrosion resistance on powder coated aluminium substrates

corrosion resistance results – best results

Corrosion test Surface Result Infiltration

Acetic AcidSalt Spray TestDIN EN ISO 9227

FiliformcorrosionEN 3665

Al - sheet / Profile5005 / 6060

Al - sheet 5005

1000 h none

1000 h F < 0.3

the good things about it :• not toxic, no hexavalent chrome•corrosion resistance (after powder coating) as

chromating• simple waste water treatment

Chrome-free pretreatment

attention points• layer is colourless• system is sensitive • layer is very thin• excellent rinsing technique is

necessary• accurate bath follow-up needed

Chrome-free pretreatment

Waste water and disposal• no decontamination necessary• no decolouring needed• simple neutralisation with Ca – salts (lime

milk)• followed by flocculation and filter press• removing fluorides by formation and

precipitation of calcium fluoride

Conclusion • It‘s only a matter of time, sooner or later, we need to

adapt ourselves to safer areas to protect ourselves,our environment and the future generations.

• There are many good proven systems to replace theChrome process for our industry and it‘s our socialresponsibility and discipline to follow.

• We nurture the good things our predecessors leftfor us and we must leave the best things for thecoming generations for them to live better.

Thank you for your attentionAny Questions?