Critical Issues for Chrome-free Pretreatment of Aluminium Alloys

Critical Issues for Chrome-free Pretreatment of Aluminium 

Alloys 

Geoff Scamans 

Innoval Technology 

International Conference on Environmental friendly Pretreatments fo Aluminium and other Metals, Oslo, Norway, June 16-18, 2004

Critical Issues for Chrome-free Pretreatment of Aluminium Alloys 

Acknowledgements 

• FICARP group 

• Andreas Afseth 

• George Thompson and Xiarong Zhou 



Two different modes of filiform corrosion: 

1: Rapid, superficial anodic undermining / surface active corrosion 

2: Slow, successive pitting caused by copper et al enrichment on the surface 

24 hours 96 hours 250 hours 500 hours 1000 hours 

20 mm 



Deformed Surface Layers 

• Since 1996 a wide range of sheet alloys have been examined and 

ultra-fine grain sized surface layers have been found on almost 

every sheet surface examined to date 

• Deformed surface layers are a generic feature of aluminium rolled 

products. Typical grain sizes are of the order of 50 nm and the 

layers contain oxide inclusions, carbides and entrapped lubricants 

• The layers are formed as a result of high shear deformation during 

rolling, particularly hot rolling, and other mechanical working 

processes such as machining and grinding 

• The layers are nano-materials with a high density of grain 

boundaries 



Block Cast AA3005 Stand 1 Hot Mill 

SEI 

BEI 

5 µm 

5 µm 

500 nm 

200 nm 



SEI 

Block Cast AA3005 Stand 6 Hot Mill 

BEI 

5 µm 

5 µm 

200 nm 

100 nm 



AA5754 H16 Architectural Sheet as Cold Rolled and after Annealing 

58% Cold rolled 100 nm Annealed 2 hours at 280°C 100 nm 



Deformed surface layers on AA6111-T4 Automotive closure sheet 

200 nm 

100 nm 



Deformed surface layers on AA5754-O Automotive Structural Sheet 

200 nm 

100 nm 



Deformed surface layers on AA5182-O Inner Panel Automotive Sheet 

200 nm 

50 nm 



Mechanically Ground Surface Layers 

• Produced over a wide range of grit sizes and lubrication conditions 

• Can be removed by diamond polishing 

• Similar surfaces produced by machining of AA7xxx alloys 

• Less stable than rolled surface deformed layers and can be 

annealed out 



AA6111-T4 Cleaned and Mechanically Ground 

180 grit 220 grit 500 grit 

500 nm 

100 nm 



AA3005 hot rolled, ground 60 grit SiC 

60 grit 

60 grit Annealed 

1µm 

500nm 

100 nm 

100 nm 



Activation of Deformed Surface Layers 

• Promoted by preferential precipitation compared to the bulk due to 

enhanced diffusion 

• Most studies have concentrated on manganese. Activation level 

depends on time and temperature of heat treatment. 

• AA6xxx alloys are probably activated by preferential precipitation 

ageing precipitates during paint bake or by natural ageing 

• AA5xxx alloys can be activated by sensitisation treatments 



Twin Roll Cast AA3005 24h Lockheed tested samples 

Effect of annealing temperature on final gauge, hard rolled material (acetone degreased). 

As rolled 200°C 250°C 300°C 

325°C 350°C 375°C 400°C 



Twin Roll Cast AA3005 after annealing at 2mm gauge 

surface layers corroded in microtome water bath 

200 nm 



Twin Roll Cast AA3005 after annealing at 2mm gauge 

0.1 mM Na 2 CrO 4 in microtome bath prevents corrosion 

200 nm 



Strain enhanced precipitation in AA3005 

Cast + 450°C 4 hrs 

34.0% IACS 

1µm 

Cast CR + 450°C 4 hrs 

44.1% IACS 1µm 



TRC AA3005 2mm sheet after annealing 

AlMnSi 

AlFeMnSi 

AlMnSi 

50 nm 



FEG-SEM characterisation progressively GDOES sputtered AA3005 CC 

3 seconds (~ 100 nm removed) 

As cold rolled 

5 seconds (~175 nm removed) 

30 seconds (~ 1 µm removed) 

Annealed 

1 µm 



Twin roll cast AA3005: Dispersoid particle density 

Particle density [µm -2 ] 

10 

9 

8 

7 

6 

5 

4 

3 

2 

1 

0 

As rolled 

Annealed 350°C 

75 150 300 1000 

Approximate depth from surface [nm] 



Acetone degreased Z19 degreased Caustic etched 

As rolled 

As SHT 

SHT 

+500h@120°C 

1000h Lockheed corrosion tested AA5754 samples 

1 cm 



Choice of Alloy 

• One simple way to reduce susceptibility to rapid filiform corrosion is 

to reduce the manganese level in the alloy 

• Alloys like AA3003, AA3103, AA3004, AA3104 and AA3005 all 

contain >1%Mn and can be highly susceptible to fast filiform 

corrosion 

• Reduction of the manganese content to 0.5% reduces susceptibility 

(AA3105) 

• An alternative is to use low manganese AA5xxx alloys that have a 

higher inherent resistance to rapid filiform corrosion 



Effect alloy manganese content. Commercially twin roll cast materials, 

Laboratory cold rolled and back-annealed (350°C) 

10 mm 

AA3005: 1.0 % Mn AA3105: 0.5 % Mn AA5005: 0% Mn 



Lockheed corrosion tested panels TRC AA5005 annealed (2h@400°C) 

24 h 48 h 96 h 250 h 

Caustic cleaned 

+ desmut 

Acetone degreased 



Acetone degreased only 

Lockheed tested samples of AA5182-O 

Z19 mild acid cleaned (H 3 PO 4 based degreaser) 

Ridolene acid cleaned (H 2 SO 4 / HF) 

Caustic cleaned + Nitric acid desmutted 

24 hrs 48 hrs 96 hrs 168 hrs 500 hrs 1000 hrs 



Alloy Processing 

• Cold rolling can progressively reduce the thickness of the deformed surface 

layer and this can reduce susceptibility to fast filiform corrosion 

• Resistance to corrosion can be improved by increasing the transfer gauge 

thickness 

• An alternative route is to heat treat rolling blocks to precipitate manganese 

before hot rolling. The alloy then behaves like an alloy with a lower 

manganese level. 



Effect of process route (TRC AA3005). Commercially twin roll cast materials, 

Laboratory cold rolled and annealed (350°C) 

1 st pass 2 nd pass 3 rd pass 4 th pass 5 th pass BA 5 th pass AA 

Route B 

Route A 

1 st pass 2 nd pass BA 2 nd pass AA 3 rd pass 4 th pass 5 th pass 

1 cm 



Cleaning 

• This is the most critical surface finishing step 

• Active surface layers must be removed by either alkaline or acid etching 

• Alloy surfaces are immune to rapid filiform corrosion after active surface 

layers have been removed. 

• Copper accumulation must be avoided as this can enhance susceptibility to 

the slow pitting type of filiform corrosion 



Effect of Cleaning: TRC AA3105. Commercially twin roll cast materials, 

Laboratory cold rolled and back-annealed (350°C) 

Solvent degreased 

(acetone) 

Mixed Acid 

(HF/H 2 SO 4 ) 

AC Electrolytic 

(H 3 PO 4 ) 

Alkaline etch 

+desmut 

10 mm 



Acid cleaning: Increased FFC for heavily cleaned TRC AA3005 samples 

(recycled metal, Cu = 0.14 %) 

Mildly Cleaned Optimally cleaned “Over” cleaned 

AC Phosphoric 

Mixed Acid 



Acid cleaning: Increased FFC for heavily cleaned samples 

Mildly Cleaned Optimally cleaned “Over” cleaned 

Al 

O 

Cu 

Intensity [arb. units] 



0 2 4 6 

Time [sec.] 

0 2 4 6 

Time [sec.] 

0 2 4 6 

Time [sec.] 



Pretreatment 

• This main function of pretreatment is to provide good adhesion 

• The simplest way to achieve this is by using a treatment to enhance the 

natural oxide layer by anodising or by hydrothermal treatment in water or 

steam 

• Coil line treatments based on fast anodising in either sulphuric or 

phosphoric acid offer advantages in terms of speed, control and uniformity 

• Fluoroacid treatments are effective but there are issues with monitoring. 

• Adhesion promoters can be used to enhance adhesion to oxidised surfaces. 



Simple model of how improved wet adhesion may prevent 

successive pitting type filiform corrosion 

Defect in paint + 

corrosive environment 

Paint 

Metal 

Poor adhesion: 

Extensive delamination 

More metal is exposed 

New pits initiate 

Filaments propagate 

Good adhesion: 

Limited delamination 

Corrosion stops 



Filiform corrosion test results (1000 h Lockheed) : 

Samples of AA6111 given AC phosphoric acid clean + spin-coat pre-treatment of 

different silane solutions. 

No pretreatment 

Chromated 

BTSE 

Silquest A-187 Coatosil 1770 Silquest A-1100 Silquest VS-142 

10 mm 



Testing 

• Most useful information has been derived from carefully monitored sets of 

test panels on exposure sites (FICARP, Alcan, TNO/ECCA) 

• The results of these studies correlate well with the results of filiform 

corrosion tests and with certain cyclic corrosion tests like the TNO test. 

• The is generally poor correlation with the results of acidified salt spray tests 

either in terms of performance ranking or in the observed mode of corrosion. 

Low temperature ASS testing can provide good results although panels 

often develop filiform after removal from the test cabinet. 

• Generally tests derived for steel substrates are not appropriate for 

aluminium. 

• A simple form of filiform testing has been developed that can be used to 

optimise alloy and process route selection and to tune cleaning treatments 

• The exposure site condition is not reproduced in most cabinet based 

corrosion tests. 



Hoek van Holland Exposure Site Results for Lightly Cleaned AA3005 

Samples made from Recycled Metal 

2 cm 

Coil 9 Coil 10 

1 cm 

Coil 11 Coil 12 Coil 13 



Corrosion ranking 

10 

9 

8 

7 

6 

5 

4 

3 

2 

1 

0 

-1 

HvH exposure 

(50/38 months) 

ASS test 

(500 h) 

3 4 5 6 7 8 9 10 11 12 13 

Coil number 

Comparison of Hoek van Holland Exposure Site Results and ASS test Results for Lightly 

Cleaned AA3005 Samples (Average values for all coil positions) 



Preliminary Analysis of Exposure Site and Corrosion Test Results 

TNO completed and reported a study of exposure site and corrosion test results on a large set of coil 

coated steel and aluminium samples. This study included 14 different combinations of pretreatment and 

coating systems applied to four aluminium alloy substrates (A1: AA3003 H26 max 0.1% Cu, A2: same as 

A1 but different producer, A3: AA5005 H14 and A4). 

SystemAlloy Pretreatment Primer Base coat Top coat Remarks 

S3 A2 NR1 no-rinse chromate P1 epoxy 4-6 µm C1 saturated polyester 17-19 µm 

S11 A1 NR2 no-rinse chrome/phos B1 polyurethane 16-20 µm C7 PUR/PA transp. 8-12 µm 

S12 A1 NR4 no-rinse chrome/phos B2 polyurethane met 16-20 µm C7 PUR/PA transp. 8-12 µm 

S13 A1 NR2 no-rinse chrome/phos P3 acrylate/epoxy 5-7 µm C8 PVDF 80/20. 16-20 µm 

N4 A1 NR2 no-rinse chrome/phos B4 PVDF 80/20 16-20 µm C15 PVDF 80/20 transp. 8-12 µm 

N5 A1 NR3 no-rinse chrome/phos B4 PVDF 80/20 16-20 µm C15 PVDF 80/20 transp. 8-12 µm 

N6 A1 NR3 no-rinse chrome/phos B1 polyurethane 16-20 µm C7 PUR/PA transp. 8-12 µm like S11 

N7 A1 NR4 no-rinse chrome/phos B5 polyurethane met/transp. 16-20 C7 µmPUR/PA transp. 8-12 µm 

N9 A1 NR5 special no-rinse B1 polyurethane 16-20 µm C7 PUR/PA transp. 8-12 µm like S11 

N15 A1 NR8 no-rinse grey B6 PVDF 80/20 16-20 µm C16 PVDF 80/20 8-12 µm metallic 

N17 A4 NR2 no-rinse chrome/phos B1 polyurethane 16-20 µm C7 PUR/PA transp. 8-12 µm like S11 

N18 A4 NR4 no-rinse chrome/phos B2 polyurethane met 16-20 µm C7 PUR/PA transp. 8-12 µm like S12 

N19 A4 NR2 no-rinse chrome/phos P3 acrylate/epoxy 5-7 µm C8 PVDF 80/20. 16-20 µm like S13 

V2 A3 R3 yellow chromate C17 saturated polyester powder 55-110 µm 

Test panels of most systems and were exposed at three exposure sites for up to 9/10 years (Den Helder, 

Delft, Hoek van Holland). Panels were also tested using the ABC test, the Prohesion test, with and without 

QUV, and the TNO cyclic corrosion test with a weatherometer with the Xenon lamp either on or off. 

Images of the tested panels have been recently made available and the purpose of the present study was 

to make a detailed analysis of these images to correlate the results of the exposure tests with the corrosion 

test results on the aluminium substrates. 



Replicate Test Panels from 9/10 year 90º North on Den Helder site 

System N5: Alloy A1, NR3 chrom/phos, B4 PVDF base coat, C15 PVDF topcoat 



DH9N N5-1 


DH9N N5-2 

DH9N N5-3 



Cyclic TNO Corrosion Tests combined with a Weatherometer 


with the Xenon lamp off (TCD) or on (TCU) 



Results of ABC, Prohesion and Prohesion with QUV Corrosion Tests 




Results of ABC, Prohesion and Prohesion with QUV Corrosion Tests 


ABC3 N5-1 

PRO2 N5-1 

PRU2 N5-1 



Grinding and Machining 

• This has been studied in detail for automotive closure sheet applications of 

alloys like AA6016 and AA6111 

• The body-in-white undergoes rectification before finishing 

• The mechanically ground layer developed during rectification of closure 

panel surfaces is not removed during the finishing operation 

• On non-ground surfaces a high resistance to filiform corrosion is achieved 

using a fluoroacid passivation treatment. Corrosion is only observed on 

ground surfaces. 

• This is not a concern provided rectification is not carried out where stone 

chip damage is likely to occur in service. 

• Aluminium vehicles can be run through mixed metal low fluoride phosphate 

lines at high volume provided an appropriate passivation treatment is used 



500 nm 

AA6016, ground (150 + 220 grit) Low fluoride phosphating and passivation 



Zr passivation layer on micrograined surface layer 

100 nm 

AA6016, ground + Low fluoride phosphating and passivation 



Alloy AA6016, Zr/Ti flouroacid pretreatment 

500 nm 

100 nm 



Alloy AA6111 ground sample, low fluoride phosphating 

Deformed layer 

200 nm 

Passivation layer (Zr) 

50 nm 



Low Fluoride 

Alloy AA6111 250 hour Lockheed corrosion test 

High Fluoride 

Zr/Ti fluoroacid 

Unground area 

Ground area 

10 mm 



Alloy AA6111 Lockheed corrosion test after low fluoride phosphating 

Unground Area Ground Area: 150 + 220 grit 150 + 220 + 350 grit 

48h 

115h 

250h 

1000h 

10 mm 



Alloy AA6016 250 hour Lockheed corrosion test results 

Low Fluoride 

250h 

Audi system (Alodine 2840) 

250h 

Unground area 

Ground area (150 + 220) 

10 mm 



Alloy AA6111 Lockheed corrosion test after 220 grit grinding 

(no PB) 

degreased 

(PB) 

degreased 

(PB) 

Etched 2 sec. 

(PB) 

Etched 5 sec. 

(PB) 

Etched 10 sec. 

(PB) 


(PB) 


1000hrs 

250hrs 

115hrs 

48hrs 

20 mm 



Alloy AA6016 Lockheed corrosion test after 220 grit grinding 

(no PB) 

degreased 

(PB) 

degreased 

(PB) 

Etched 2 sec. 

(PB) 

Etched 5 sec. 

(PB) 


(PB) 


(PB) 


1000hrs 

250hrs 

115hrs 

48hrs 

20 mm 



Corrosion performance 

• AIV & Competitor vehicle test shows aluminium 

corrosion performance BIC 

• LWV running in Sweden for long term corrosion 

& durability since 1999 

• LWV corrosion car better than steel test vehicle 

at end of test 

• X350 production corrosion car complete 11/02 – 

no issues 

• Corrosion test validated interface between BIW 

& other vehicle systems 

Materials for Lean Weight Vehicles 

5th International Conference - 5- 6 November 2003 

M Ellis and A Tautscher 



SEM images of corrosion car sample from an area of low corrosion 

10 mm 

100 µm 

20 µm 500 nm 



SEM images of corrosion car sample from an area of corrosion 

10 mm 

100 µm 

20 µm 500 nm 



Secondary Metal 

• As part of the FICARP project we examined a continuously cast alloy 

AA6011 made from secondary metal 

• The corrosion resistance of the as-fabricated sheet was poor 

• Removal of deformed surface layers removed the susceptibility to rapid 

filiform corrosion but not copper or impurity driven filiform corrosion 

• A high resistance to filiform corrosion was achieved using a flame spray 

pretreatment to enhance adhesion 



AA6011 Lockheed As Received 1.22 mm Acetone Degreased 

0.41 Mn, 0.71 Fe, 0.8, Si, 1.16 Mg, 0.53 Cu, 0.057 Cr, 0.25 Zn, 0.046 Ti, 0.0011 Be, 

0.021 Pb, 0.006 Sn , 0.0008 Na, 0.0011 Ca 



Lockheed, NaOH/HNO 3 ~ 2 µm Removal, 1.22 mm Sheet (20 sec at 60°C) 



Lockheed, NaOH/HNO 3 + Pyrosil Flame Spray, 2 µm Removal, 1.22 mm Sheet 



Summary 

• Use of chrome-free pretreatment can be facilitated by appropriate 

choice of alloy and process route 

• In surface finishing the cleaning step is critical to remove damaged 

surface layers. In addition surface accumulation of elements like 

copper must be avoided. 

• Although chrome-free systems based on the use of fluoroacids are 

effective there are considerable advantages to be gained from the 

use of enhanced natural oxides and hydroxides 

• Adhesion promoters are effective as monolayers in combination with 

enhanced oxide films 

• Selection of appropriate test methods is critical

Critical Issues for Chrome-free Pretreatment of Aluminium Alloys

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