Microgalvanic corrosion of laser-welded HSLA steels

Microgalvanic corrosion of laser-welded HSLA steels

Looi, Y.M.

De Wit, J.H. (promotor)

Mechanical Maritime and Materials Engineering

2008-01-08

Laser welding of galvanized high strength low alloy (HSLA) steels leads to the evaporation of zinc at the weld and the formation of a heat-affected-zone (HAZ). High heat input due to welding generates macro galvanic coupling between the weld and the parent metal as well as micro galvanic corrosion at different microstructures of the weld, HAZ and the parent metal. Different microstructural substructures exhibit different electrochemical characteristics, potentially leading to microgalvanic corrosion when exposed to an aggressive electrolyte. One of the main aims of this project is to study the effect of laser welding parameter settings on the corrosion behaviour of galvanized steels. It is also important to understand the corrosion behaviour after the zinc coating is consumed or damaged. Therefore, the corrosion mechanisms in and without the presence of the zinc coating have been extensively investigated. Comparison of macroscopic corrosion behaviour of the parent metals with and without zinc, and laser welded samples were made based primarily on the results obtained from potentiodynamic polarisation measurements. As expected, the parent metal without zinc coating shows uniform corrosion when polarised in the anodic direction while samples with zinc coating indicate sacrificial protection of zinc to the steel substrate first, followed by dissolution of the steel. Microscopic characterisation of samples, when zinc was almost consumed, indicate intergranular corrosion at the HAZ. FE-SEM results indicated traces of residual zinc on the welds. The presence of zinc at the weld was not expected since zinc has lower melting point than the steel. It is likely that zinc vapour condensed on the surface. Results also showed that the weld is the most corrosion susceptible area for all samples but with ranging severity. Samples A653 welded at 1750W at 3.0m/min show the best corrosion resistance, while samples welded at 3000W and slowest welding speed of 2.4m/min suffer the most drastic microstructural change and herewith the lowest corrosion resistance. After obtaining the general corrosion behaviour of galvanized and welded samples, a microstructural characterization study on laser-welded joint was performed. The local corrosion behaviour related to the microstructure was studied using the micro electrochemical cell. It has been observed that the parent metal consists of ferrite while the weld consists of mixtures of ferrite and small amounts of Widmanstätten Ferrite (WF). Results also indicate that the volume of WF was dependent on the welding parameters used. It was found that ferrite microstructure has a more positive corrosion potential than WF microstructure. It has also been observed that the corrosion susceptibility of the welds improved when the volume of WF microstructure increased. Results obtained from Scanning Vibrating Electrode Technique (SVET) techniques also support the findings from the micro electrochemical cell measurements. In order to identify the corrosion initiation and propagation of laser-welded samples, the Difference Viewer Imaging Technique (DVIT) was used. The welded samples indicate corrosion initiation at the centre of the weld and fusion lines before propagating to the rest of the surface. The corrosion susceptibility and mechanisms of samples after zinc removal are shown to be dependent on the type of microstructure, the type and thickness of oxide formed on the surface which are influenced by the welding parameters and applied potentials. The oxide film produced in the initial stage of the corrosion processes differs for the various polarisation potentials where polarisation in the potential range of -600mVSCE to -900mVSCE, the film is very thin and not sensitive to pitting, while polarisation to the potentials range of -500mVSCE to -300mVSCE (the oxide thickness larger than 5.2nm), can become sensitive to pitting.

microgalvanic corrosion
hsla steels
yag laser welding

http://resolver.tudelft.nl/uuid:0e09d7c5-00ba-4180-9988-f047ae1b9e48

978-90-77172-36-0

Institutional Repository

doctoral thesis

(c) 2008 Y.M. Looi