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Numerical modelling / lifetime prediction of delamination polymer coating disbonding and material degradation

Final Report Summary - DEGRADATION MODELING (Numerical modelling / lifetime prediction of delamination polymer coating disbonding and material degradation)

The project 'Degradation modelling' aimed to develop mathematical models to predict the lifetime of materials which undergo under paint or confined zone corrosion. Such a work will contribute to the faster design and validation of products. The project is structured into six work packages:

WP1: corrosion mechanisms databases
WP2: under paint corrosion simulation
WP3: confined zone simulation
WP4: Interface simulation by ab-initio methods
WP5: Experimental refinement of input mechanisms
WP6: Synthesis and project management

Project objectives and major achievements

This project sought the development of numerical models for the prediction of material degradation by underpaint and confined zone corrosion for specific mechanisms of degradation. The simulation models developed in this work will be used to predict how specific mechanisms of corrosion will be affected by surface and climatic parameters. Through the use of numerical experimentation, we will determine the sensitivity of corrosion rate to various surface and environmental factors for a given mechanism. This will reveal trends in product performance that can be used to target the development of new products for given climatic situations.

Five main work-packages were defined to succeed in developing reliable corrosion models:

WP1. Database of corrosion mechanisms
The aim of this WP was to create a database and a review of the corrosion mechanisms underfilm corrosion and confined zone corrosion. Parameters will include the material, surface treatment, ED-coating (type, thickness), eventual sealer, topcoat, climatic parameters, etc.

WP2. Under paint corrosion simulation
The specific objective of the modelling effort will be to simulate the 'salt spray test' for a hypothetical painted galvanised steel substrate defined by various input parameters and an experimental geometry.

Limiting the work to the salt spray test is justified since this test is still the industrial standard for the building and home appliance markets. In this way, we had to take into account wet / dry cycles that are used in most automotive corrosion tests greatly simplifying the theoretical program.

WP3. Confined zone simulation
Numerical simulation of confined zone corrosion will be based on simplified models including thermodynamics and kinetics of the process as well transport equations of species in the system.

WP 4. Interface simulation by Ab initio methods
The objective of this WP was to provide structural input models (atomic coordinates) of the respective interfaces, which can be used in future spectral calculations, in the investigation of reactions at the interface and for the modelling of diffusion processes. The obtained structure and binding energies at the interface can be included as input information / parameters in large scale empirical models.

WP 5. Experimental refinement of input mechanisms
Delamination experiments, as previously described, were performed on the thiol modified Au(111) surface and paraffin or solid long-chain alkane films so as to obtain fundamental rate constants and kinetic data for input into the empirical model.

Level of achievement at the end of project life

At the end of the project life, the following achievements have been reached:

WP1: Databases for delamination of automotive materials and building materials as well as for confined zone corrosion have been produced.The database for delamination of automotive materials have 739 different cases/samples with different combinations of material parameters, exposure conditions and climate parameters. The database for delamination of buiding materials contains 395 cases / samples (not counting sample replicates) and the database for confined zone corrosion 208 cases / samples.

WP2: Three underpaint corrosion models simulating different systems configurations have been produced in Comsol Multiphysics environment. The two first models apply for zinc-polymer systems for which cathodic delamination govern the electrochemical reactions at the buried interface. In the first model, the elemental phenomena that control underpaint delamination have been detailed and translated into mathematical equations on the basis of Allahar pioneering work. In the second model, the geometry of the blister has been modified so as to be more representative of industrial corroded samples. This model better simulates the delamination of zinc-polymer systems and the results of simulations are consistent with confidential – reserved to project consortium members literature data. The last model simulates the paint delamination at the cut edge of zinc coated steel. The model integrates new developments based on anodic delamination mechanism and part of the work already performed on cathodic delamination modelling. The results of simulations are in good agreement with WP1 database tendencies and new experimental data obtained from delamination trials performed in WP2.

Furthermore, the links between coating thickness and delamination rate are clearly evidenced for real painted systems, giving a scientific explanation to experimental results generally observed on painted galvanized products. This last model can be considered as a basis for further developments aiming at simulating with more accuracy the corrosion of real systems in natural environment or in accelerated cyclic corrosion tests.

WP3: The four models simulating the confined zone corrosion in cyclic corrosion test for pure zinc and galvanised steel substrates have been completed. The results of model simulating confined zone corrosion without drying are in good agreement with the general corrosion behaviour and the distribution of corrosion products in the crevice observed empirically. Critical experiments have been performed which included mass loss during drying, analysis of the distribution and type of corrosion products, pH changes and the effect of temperature.

The final model clearly identifies and quantifies the importance of the various processes during corrosion of zinc and galvanised steel in a confined zone. Model calculations were performed with galvanised steel where the corrosion was allowed to penetrate the outer layer of zinc. Situations where penetration was allowed to occur under stationary wet conditions as well as under wet-dry cycles were studied in some detail. Conclusions of industrial as well as some scientific importance were drawn from the model results. Nevertheless, the model as implemented here in Comsol Multiphysics is very demanding and requires manual interference. The calculations necessary for three wet-dry cannot be completed in less than one week even on a power full multi-processor computer. Alternative formulations are possible in Comsol Multiphysics and may have to be explored if many different conditions are to be studied. Additional experimental investigations are necessary to confirm findings obtained so far and for refinement of numerical models.

WP4: Structural atomistic models for 2 thiol / gold interfaces were generated (methyl- and ethyl-thiol) and have been presented electronically in a small model (surface unit cell) and an expanded periodically repeated model. Subsequently, models for propyl- and butyl-thiol on gold were developed, as well as alternative structures based on gold-adatoms and vacancy reconstructions. The structures and binding energies for all these models were determined as potential input data for larger scale simulations. Two theoretical approaches to simulate ion incorporation and diffusion / migration at a buried interface have been implemented and applied to a paraffin / thiol-SAM / gold model interface).

In addition to the deliverables defined for WP4, an advanced methodology to model electrochemical processes such as oxygen reduction in underpaint corrosion has been developed and tested successfully.

The reactive intermediates formed in the oxygen reduction may be a key to understand the reaction cascades that ultimately lead to deadhesion.

WP5: For the studied system, kinetic parameters were determined for oxygen reduction at these surfaces as well as for diffusion / migration of cations along the interface between the thiol SAM / Au(111) and the model coatings (paraffin or PVB). Diffusion constants and kinetic input data were obtained.

Problems and corrective actions

At the end of year 3, the level of technical achievements is consistent with the project objectives and the scheduled work program. All deliverables have been produced with a good degree of fulfilment regarding the acceptance criteria.

Dissemination of the knowledge

Several ways are employed for the dissemination of the knowledge obtained in the frame of this project:

- web site for corrosion databases
- publications in scientific journals
- promotion of corrosion softwares by Comsol Multiphysics
- communication to industrial steering committee.

The databases for paint delamination in automotive and building corrosion and for confined zone corrosion will be available for public download at Swerea KIMABs website. A user friendly interface will be included in the software package which is downloaded. The database will be maintained at Kimab website during at least 4 years after the end of the project.

Because the results of this project are for the most part of a scientific character, a major method of disseminating the knowledge is the publication of significant results in either peer-reviewed journals or in international conferences.