Oxide coatings as corrosion inhibitors to improve the durability of adhesively bonded Al alloys

Transcription

1 AETOC , Billerbecker, Germany Oxide coatings as corrosion inhibitors to improve the durability of adhesively bonded Al alloys Forschungsfeld - Jianmin Shi, Helger Pries, Klaus Dilger Flexible Fahrzeugkonzepte und Fahrzeugproduktion Schweißtechnik, Technische Universität Braunschweig j.shi@tu-braunschweig.de Tel:

2 Outline Introduction and motivation Manufacture of light-weight structures, Corrosion and failure of adhesive joints Electrochemical measuremtns as a means to evaluate adhesive joints Oxide conversion coatings for adhesive bonding of Al alloys Ti-oxide, Zr-oxide, Cr 3+ -oxide Sol-gel coatings for adhesive bonding of Al alloys Ce-oxide Summary Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 2

3 Light-weight vehicles: a multi-materials approach Increasing use of light-weight materials in design High-strength steels Aluminum alloys Magnesium alloys Fiber-reinforced plastics: CFP, GFP Light-weight vehicle concepts (Ford, Mach-I/II) Joining technologies in manufactureing processes Mechanical fastening Welding Adhesive bonding Direct bonding: Pressure /Temperature Adhesive bonding of CFP body parts and Al-frame (BMW i3) Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 3

4 Adhesive bonding Adhesive bonding: a technique to join materials together using adhesives, particularly, for joining different materials Structure of adhesive joints: Substrate 1 Adhesive Interfaces/ boundary regions Substrate 2 Applications: aerospace-, construction-, automotive-industry etc Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 4

5 Druability and failure of adhesive joints Durability and integrity of adhesive joints: withstand various loads under service condictions, function well without failure for a desired service life Failure of adhesive joints: Unsuitable material-choice, geometrial design and surface-preparation Fault/error during manufacturing processes Chemical/electrochemical reactions at interfaces due to enviorenmental factors: H 2 O, Cl -, etc. Temperatur, Pressure Corrosion at the interface between adhesive and Al alloy Mechanical stress etc Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 5

6 Evaluation of adhesive joints using electrochemical methods Motivation: Evaluation of adhesive joints and understanding degradation/ failure mechanisms Screening surface-preparation: corrosion parameters (OCP, DP, EIS) Durability/degradation of adhesive joints: Water uptake and diffusion in adhesives Delammination of adhesive layers Chemical/electrochemical reactions at interfaces Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 6

7 High-Pressure Die Casting (HPDC) of Al Alloys Characteristics: low cost, high production rate, near-net shap components HPDC cycles: 1. Spraying of lubricants 2. Filling of shot sleeve with melt 3. Pressing the melt into die Surface contamination by lubricants Temperature gradient across the casting 4. Removing of casitngs Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 7

8 Chemical conversion coatings for adhesive bonding of HPDC Al alloys A universal chemical pretreatment process for adhesive bonding of HPDC Al alloys. HPDC Heat treatment grit blasting Wiping with MEK Degreasing in NaOH solution Deoxidation in F - -containg H 2 SO 4 solution Conversion Coating: Oxide Effectivness of oxide coatings to improve the durability of adhesive bonded HPDC Al Replacement of Al-oxides by oxides: Zr-, Ti-, Cr 3+ -oxides Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 8

9 Surface characterization of HPDC Al plates Quantitative determination of near-surface chemical compositions, Electron Probe Micro-Analysis (EPMA) EPMA measurement: Equipment: Jeol 8200 Quantitative analysis from B to U Point analyis and surface mapping Analysis limit: 0,01 bis 0,001 mass % Spacial resolution: 1-3 µm Depth information: ca. 1 µm 250 nm 200 nm 150 nm 100 nm 50 nm Measuring limit Residual die lubricants on casting surfaces can be reflected by C-containing species which are transformed into an equivalent C-film thickness in the measured point 10 x 10 um The average C-film thickness and spacial distribution in an area of 1 mm x1 mm The distribution of alloying elements can also be measured in a similar way Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 9

10 Chemical inhomogenity of HPDC Al surfaces Quantification of residual lubricants Location-dependent distribution of Cu Polysiloxane Polysiloxane + wax EN AC-AlSi9Cu3(Fe) Near melt-ingate (AN) Away from melt-ingate (AF) The amount of residual lubricants and the distribution of alloying elements depend on locations, chemical compositons and dilution ratios of die lubricants, as well as on cooling methods Localised corrosion after immersion test (5 mm CuSO 4 ) Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 10

11 Evaluation of adhesive bonding properties Single lap shear test with and without aging: DIN EN 1465 Evaluation criteria: 1. single lap shear strength (MPa) 2. qualitative analysis of failure modes Wedge test without/with aging: DIN Evaluation criteria: 1. Initial crack length (mm) 2. Crack extension (mm) (after aging) 3. Failure mode and the percentage of cohesive failure (%) Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 11

12 Adhesive bonding of as-cast HPDC Al alloys Polysiloxane, 1:125, air Cohesive failure (%) Initial crack length (mm) Polysiloxane+wax, 1:125, air Polysiloxane, 1:50, air Polysiloxane+wax, 1:50, air Wedge test DIN Substrate: EN AC-AlSi10MnMg Cooling in air Adheisve: 1P Epoxy Curing: 180 o C, 30 min Aging: no The initial bonding properties correlate with residual lubricants for the application ratio of 1:125, measured using EPMA For the application ratio of 1:50, the failure mode is dominated by adhesive failure, thus no difference could be observed between the two die lubricants Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 12

13 Effect of die lubricants on initial bonding properties: Failure modes AF AN SL1697S, 1:125 SL3188, 1:125 SL16195S, 1:50 SL3188, 1: Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 13

14 Effect of conversion coatings Degreasing Deoxidation Conversion coating Designation R A S2 F - -containing H 2 SO 4 solution K1 Ti-oxide K2 Zr-oxide K3 Cr(III)-oxide GZ (Gusszustand) R x RA x x RA-S2-K1 x x x x RA-S2-K2 x x x x RA-S2-K3 x x x x Step Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 14

15 Effectiveness of pretreatment processes: Effect of conversion coatings Wedge test DIN Substrate: EN AC- AlSi10MnMg SL 3188, 1:125, Surface: various Adhesive: 1P Epoxy Curing: 180 o C, 30 min Aging: 70 C, 5 cycles The three oxide coatings are effective to improve the durability of adhesive joints: K1: Ti-oxide, K3: Cr 3+ -oxide, K2: Zr-oxide Ti-oxide coating shows superior durability and will be used in the conversion step Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 15

16 Effectiveness of pretreatment processes: Fracture surfaces of adhesive joints As-cast Grit-blast Grit-blasting + RAS2K1 (Ti-Oxid) Crack extension Grit-blasting + RAS2K2 (Zr-Oxid) Grit-blasting + RAS2K3 (Cr III -Oxid) Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 16

17 Confirmation of the effectiveness of pretreatment processes by electrochemical impedance spectroscopy Z im (Ohm cm 2 ) HPDC AlSi9Mn as-cast only echted echted+zr-oxide Z re (Ohm cm 2 ) Equivalent circuit: Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 17

18 Applicability to other HPDC alloys Wedge test: DIN Die lubricant: wax-containing Surface: as-cast and chem. pretreated Adhesive: 1P Epoxy Curing: 180 o C, 30 min Aging: 70 o C, 5 wt% NaCl solution Durability in as-cast state depending on alloy compositions: EN AC AlSi9Mn > EN AC AlMg5Si2Mn > EN AC AlSi10MnMg > EN AC AlSi9Cu3(Fe) No dependence of durability on alloy compositions after chem. pretreatment Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 18

19 Ce-oxide coatings for adhesive bonding of Al alloys Preparation of Ce-oxide coatings for adhesive bonding of Al alloys Chemical conversion processes: Deposition of Ce-oxide in a solution of Ce(NO 3 ) 3 + (NH 4 ) 2 S 2 O 8 (APS) or Ce(NO 3 ) 3 + H 2 O 2 (H 2 O 2 ) APS H 2 O 2 Chemical conversion process Sol-gel process: Formation of sol (Ce(NO 3 ) 3 + citric acid + ethanol) Dip coating Drying at 80 o C (SG80C) or 200 o C (SG200C) Dip coating process Master Thesis: Bo Yang, Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 19

20 Surface characterization: SEM and EDX Substrate: HPDC AlSi9Cu3(Fe) APS SG80C SG200C Formation of Ce-oxide coatings Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 20

21 Electrochemical characterization: Dynamic polarization Substrate: HPDC AlSi9Cu3Fe, (A226) Corrosion resistance: SG200C > H 2 O 2 /APS > SG80C > as-cast Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 21

22 Electrochemical characterization: Electrochemical impedance spectroscopy Substrate: HPDC AlSi9Cu3Fe, (A226) Zim(Ohm*cm 2 ) As-cast APS H2O2 SG80C SG200C Zre (Ohm*cm 2 ) Corrosion resistance: SG200C > H 2 O 2 /APS > SG80C > as-cast Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 22

23 Durability of adhesive joints: wedge test Substrate: HPDC AlSi9Cu3(Fe), Adhesive: 1K epoxy, Aging: 7 cycles Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 23

24 Durability of adhesive joints: Fracture modes Substrate: HPDC AlSi9Cu3(Fe), Adhesive: 1K epoxy, Aging: 7 cycles As-cast APS H2O2 SG80C SG200C Crack length Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 24

25 Summary and outlook Surface coatings can be prepared using different methods on Al substrates, including chemical conversion processes, sol-gel processes Various oxide coatings can be introduced in surface-preparation steps to replace Al-oxides for adhesive bonding of Al alloys and can substantially improve the durability of adhesively bonded Al alloys The durability of adheisve joints correlates to the improved corrosion resistance of Al-alloys, but the improvment of corrosion resistance can not ensure a durable adhesive-joints Optimization of the sol-gel process for Ce-oxide coatings and incopration of organic inhibitors to improve corrosion resistance and durability of adhesive joints shall be considered in the future work Combined measurements of optical absorbtion and electrochemical impedance under aging condtions will be performed in the future Oxide coatings as corrosion inhibitors to improve the druability of adhesive joints Jianmin Shi page 25

26 The work partially supported by BMWi (IGF N ), DFG (SH802/2-1) and contributed by students in their thesis work Thank you very much for your attention!