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Thin Organic Coatings (TOCs)

Description

Thin organic coatings offer very high corrosion resistance based on a barrier effect. They are designed to retain good weldability by means of metallic particles included in their organic matrix.

Lubricants are also present in the resin to improve drawability. The chemical affinity between the main structural adhesives and the TOC surface increases the durability of adhesive bonds, even after ageing.

Applications

Steels with organic coatings are protected from corrosion, have a high-quality surface and can be readily shaped and welded. They are therefore recommended for numerous automotive applications. The single-sided version can be used to make both visible and non-visible parts. The double-sided version is used for non-visible parts.

TOCs for the automotive industry are specially designed to increase hollow body corrosion protection. They can help reduce the use of additional protection measures such as wax or mastics. They can also improve protection in hollow areas that are difficult to protect by cataphoresis and substantially reduce design costs. The main applications are vehicle closures, body sides, under bodies, shock absorbers and the full range of hollow body beams. These products are designed to meet automobile body manufacturers' requirements in terms of reducing the cost of anti-corrosion guarantees.

Technical characteristics

Thin organic coatings are applied over a metal coating. They can be applied to one or both sides.

The ArcelorMittal product line is composed of first and second generation organic resins in conjunction with a surface treatment.

The surface treatments used by ArcelorMittal are now chromium-free.

Substrate

Coating

Target corrosion performance

1st generation
Thin Organic Coatings

Zn electrogalvanized sheet (pure zinc)

Chromium-free surface treatment + 2.5 to 4.5 µm organic coating containing conductive particles

10 cycles of accelerated VDA corrosion test without appearance of red rust

2nd generation
Thin Organic CoatingsZn electrogalvanized sheet (pure zinc)Chromium-free surface treatment + 3 to 5 µm organic coating containing conductive particles20 cycles of accelerated VDA corrosion test without appearance of red rust

Other thin organic coatings are currently being industrialized.The goal is to offer, in the short term, an optimized substrate - surface treatment - organic coating system using new generations of chromium-free surface treatments (in 1 or 2 steps) to further enhance product corrosion performance.

First generation type organic coatings are compatible with all substrate qualities except bake hardening steels. Second generation type resins are compatible with BH steels.

Morphology

Cross-section of a first generation TOC containing conductive particles

Scanning electron micrograph of the surface of a first generation TOC

Recommendations for use and secondary processing

The TOC layer substantially increases protection against pitting corrosion.

In lock seam configuration, TOC applied to 5/5ì electrogalvanized steel withstands 10 cycles of VDA 621-415 accelerated corrosion testing without the appearance of red rust.

This makes it possible to reduce the use of additional protective measures in hollow bodies and lock seams.

Sheet coated with organic coatings can be readily formed in chrome-plated tools.

The figure below shows an example of a plane/plane friction curve on an oiled TOC.

Measured friction coefficient of a TOC coated sheet

These products can be spot-welded. To lengthen electrode life, direct contact between the TOC and the electrode should be avoided (single-sided organic coatings).

Based on its experience in characterizing these products for spot and laser welding, ArcelorMittal can provide technical assistance in adjusting the welding parameters to all commercially available organic coatings.

These products can be joined mechanically and by adhesive bonding. They are compatible with most structural epoxy adhesives used in the automotive industry.

Products can be phosphated and painted at the customer's premises using standard processes.  They lend themselves to cataphoretic painting with excellent paint adhesion.

Please consult us for additional information concerning specific adhesives and mastics.

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