Corrosion proceeding review (2016 international congress and exhibition on aluminium brazing). Presentation L. peguet

Development of an electrochemical depth

profiling method to optimize the corrosion

protection on aluminium cladded sheets for

automotive heat exchangers

L. Peguet, V. Lair, E. Mendez, H. Noui

C-TEC Constellium Technology Center, Voreppe, France

ASST 2015 - Madeira, May 20th

Electrochemical depth profiling method is a “must-have” tool to

optimize corrosion resistance of brazing sheets

Aleris

SAPA

Sumitomo LMI

Furukawa Sky

M2I Delft

Need for an electrochemical depth

profiling tool at Constellium C-TEC with

the best accuracy and robustness.

To design our new mono- & multi-clad innovative brazing sheets.

Choice of a method based on successive etching

4 steps:

Reach the relevant depth: Chemical

pickling in NaOH 2M.

Choice of a surface cleaning

procedure (chemical, mechanical,…)

Choice of the relevant environment for

electrochemical potential measurements

Application to mono or multiclad

brazing sheets to define their ID cards in

terms of potential profile.

Smut

Core

Clad 1

Clad 2

V

ref

V

ref

Potentiel

Depth

Experimental setup

Pickled area

Potential measurement area

cm

Solution : NaOH 2 mol/L

Temperature : ambiant→40°C

Surface cleaning performed in

acetone-ethanol mixture aided

by ultrasonic treatment in order

to remove the smut layer

Chemical pickling Electrochemical potential measurement

Pickling depth

Choice of the relevant environment to measure the electrochemical potential

- Example of the ASTM G69 standard -

ASTM G69 solution is widely used to measure the electrochemical potential of bulk

aluminium alloys but is very severe when considering electrochemical potentials at

a given depth on thin-gauge brazing sheets.

Top view - Binocular Microscope Cross section - Optical Microscope

Localized attack on a 10min-pickled as-rolled 3916-4045/270m/H24 specimen after

a 1h-exposure in NaCl 1M + H2O2 9ml/L (ASTM G69 standard).

Penetrating

pits


- Necessity of chloride-based solution -

As-rolled 3916-4045/270m/H24 specimen after a 10min pickling (etching depth not exceeding

the 4045 layer thickness).

3h-immersion time was carried out.

Chlorides are definitively required in order to pin a stable potential corresponding

to the so-called “pitting potential”.

(N2 deaerated solutions)

Po

ten

tia

l (m

V/S

CE

)

Time (h)


- Influence of aeration -




Signal stability is improved by solution aeration as a stronger cathodic reaction

helps to pin the pitting potential.

Po

ten

tia

l (m

V/S

CE

)

Time (h)

NaCl 0,1M

aerated

NaCl 0,1M

deaerated


- Influence of acidification -




Signal stability is again improved by using an acidic solution (HCl 0.1M) allowing a

fast depassivation. A 30 min measurement is then sufficient to get a stable potential.

Po

ten

tia

l (m

V/S

CE

)

Time (h)

(aerated conditions)


- The case of a Cu-free material -

3xxx(0%Cu)/4343/330m/O after brazing. Investigated pickling depth is 65m corresponding

to the free-Cu core alloy.

30min-immersion in HCl 0.1M + increasing additions of H2O2 .

When the core alloy to be investigated does not contain copper, adding H2O2 may be

necessary. A HCl 0.1M + 9mL/L H2O2 solution is to be chosen.


- Comparison with ASTM G69 solution -

3916-4343/400m/O after brazing:

Core alloy (after 60min etching at 40°C)

Clad (after 5min etching at 40°C)

Faster signal stabilization.

Better resolution in terms

of potential difference

between the core alloy and

the clad

30 min electrochemical potential measurement in HCl 0.1M + H2O2 9mL/L compared to ASTM G69

NaCl 1M + H2O2 9mL/L standard solution

ASTM G69 standard.

Examples of electrochemical depth profiling applications

- Brazing sheet including a Zn-containing clad: a “case study” -

3915-7072/270 m/H24 before / after brazing.

30 min electrochemical potential measurements

in HCl 0.1M after pickling for different durations.

(Each measurement is repeated twice)

Before brazing: 150mV difference is

evidenced between the zinc-containing

surface and the 3916 core alloy

60-min pickled surface after

electrochemical measurement

10-min pickled surface after

electrochemical measurement


- Brazing sheet including a Zn-containing clad: a “case study” -

3915-7072/270 m/H24 before / after brazing.

30 min electrochemical potential measurements after pickling for different durations.

Before brazing:

=150mV.

EPMA analysis

After brazing:

=50mV.


- Influence of the brazing treatment -

3916-4045/400m/H24 before / after brazing.

Electrochemical potential profile measurement in HCl 0.1M+H2O2

After brazing: A 40mV

difference is evidenced

between the surface and

the 3916 core alloy


- Influence of the brazing treatment -

3916-4045/400m/H24 before / after brazing.

Electrochemical potential profile measurement

in HCl 0.1M+H2O2

Cu phases are re-solutionized during brazing

leading to a 70mV increase of the core potential.

EPMA profiles measured from surface to mid-thickness

V=70mV


- Influence of Cu addition in the core alloy -

Cu-containing core alloy: 3xxx(0.6%Cu)/4343/330m/O after brazing.

Cu-free core alloy: 3xxx(0%Cu)/4343/330m/O after brazing.


V=40mV: “state-of-the-art” surface

sacrificial corrosion protection.

Cu-free core: Flat electrochemical

potential profile

After a 4 weeks SWAAT test

Cu-free sheet after brazing


- Profiling on multilayer solutions -

3916-3xxx-4343/480m/O after brazing.


V=60mV: Optimized sacrificial

corrosion protection.

The additional interlayer leads to an extended Cu diffusion profile from the 3916 core to the

extreme surface resulting in a thicker low-Cu surface band which offers an optimized sacrificial

behavior of the top layers.

Clad + interlayer

A methodology to determine the depth-electrochemical potential profile by successiveetching of cladded aluminium sheets was optimized. The protocol includes threestages:

A chemical pickling in 2M sodium hydroxide in a dedicated flat cell.

A surface cleaning in order to remove the smut layer.

A 30min measurement of the electrochemical potential in 0.1M HCl (+H2O2)using a suitable electrochemical cell.

Conclusions

Examples of applications were given on state-of-the-art long life products as well ason a multilayer solution:

Dominant protection mechanism driven by copper gradient.

Optimizing this gradient (copper content/re-solutionizing of precipitates)is of first importance.

Sacrificial property of an additional interlayer in multilayer productsmakes it a reference material for more demanding applications.

Automotive

Corrosion proceeding review (2016 international congress and exhibition on aluminium brazing). Presentation L. peguet