White Rust Guideline - September, 2014

R&D PROCESS Confidential information Page 1 PHD ENGINEERING Copyright 2014 Gestamp

GUIDELINE FOR WHITE RUST IN

USIBOR 1500P AS150 AND AS80

COILS AND SHEETS

Date: September, 2014 Version: 00 Issued by: PHD Engineering Approved: M. Lopez


INDEX

1. Purpose 3

2. Background 3

3. Effect of white rust 4

3.1 Effect into the AlSi coating properties 4

3.2 Corrosion after press hardening 5

3.3 Paint adhesion 5

3.4 Weldability 6

4. Customer requirements 6

5. When to scrap incoming raw material 6

5.1 Flat sheets stacked in packages 7

6. How to avoid white rust 7

7. Micro oiling 7

8. Quality control 8

9. References 8


1. Purpose

Increase knowledge about white rust at Gestamp plants and how to deal with it and possible claims from the customers.

2. Background

White rust or white oxidation is a surface condition for AlSi coated sheet metal. Aluminium coated products exposed to air will

almost instantly form a thin (nm) coat of aluminium oxides (AlO and AlO) that stops further oxidation. This is normal for all Al

products.

Picture 1. White rust

White rust might occur in AlSi coated blanks when the blank is exposed to water or moist or even high relative humidity. The

effect can be accelerated when the oxygen supply is limited, so called crevice corrosion. Such conditions can be fulfilled in a

coil or high stacks of blanks if theres moist between the layers. White rust cant be cleaned, and will be visible after

hotstamping, see picture 5.


Root causes

White rust can appears in both, blanks and coils. Root causes to find white rust into our incoming material could be:

- During material transportation or storage:

The water can leaks directly over the blanks/coils coming from the rain, from the top of the truck, from puddles of

water, etcAlso could occurs when loading/unloading the material from/to the truck, changing the material from cold to a

warmer ambient.

- Moisture condensation over/in between the blanks/coil:

Water vapour condensates on top of the surface of a metal if the temperature of the blank/coil drops below the dew

point of the surrounding air.

3. Effect of white rust

The surface properties are slightly different from parts without white rust. The main affected parameters due to white rust are

paint adhesion and weldability. Gestamp PH production processes such as heating and forming are normally not affected.

3.1. Effect into the AlSi coating properties

The raw material affected by white rust before press hardening has no big differences in terms of metallography compared

with material without white rust (see pictures 2 and 3, [Azterlan, April 2013]). The only difference we can see on the surface

coating is slightly higher oxygen content. The material properties are not affected (interdiffusion layer, total coating high and

martensitic structure under OEMs standards)

Picture 2. 22MnB5 + AS 150 blank micrograph before press hardening (left side, blank without white rust / Right side, blank with white rust)

Picture 3. 22MnB5 + AS 150 part micrograph after press hardening (left side, without white rust / Right side, with white rust)


3.2. Corrosion after press hardening

A corrosion test has been done following UNE EN ISO 9227, after 24hours test; there are no big differences on the oxidation

process between white rust parts and parts without white rust. (See picture 4 [Azterlan, April 2013])

Picture 4. Press hardened parts after corrosion test (without painting)

(left side, part with initial white rust/ right side, part without initial white rust)

3.3. Paint adhesion

According to the report [Arcelor, November 2012], white rust may slightly deteriorate the paint-adhesion. Our internal

investigation done by our Gestamp R&D technicians has not been able to duplicate the results from Arcelor. According to our

test, the paint adhesion is still within some customers (Volvo) requirements. The test could not show any degradation in paint

adhesion, see pictures 5, 6 [Hkan Andersson, June 2013]

Picture 5. Press hardened part with white rust Picture 6. Painted part with white rust

Surface affected

with White rust

Usual Surface

without white rust


3.4. Weldability

According to different studies done by Arcelor [Arcelor, November - 2012] and Gestamp R&D [Hkan Andersson,

September 2013] the white rust shall not impact significantly the resistance spot-weldability even if the transition-resistance

is increased in the defect-areas. As summary from both reports we could confirm the following lines when welding material

with white rust:

Higher transition resistance in parts

Bigger transition resistance variations

Higher energy dissipation during welding

No significant difference in cross section appearance and in weld nugget diameter

The weld joint strength is equal with or without white rust

Electrode caps are more affected welding blanks with white rust

4. Customer requirements

Its important to check customer requirements before sending any parts with white rust. In example, Gestamp HardTech is

now allowed to send parts to Volvo as all their requirements are fulfilled. Audi and VW are still rejecting parts with white rust.

Please check locally your customer requirements.

5. Incoming raw material

If the white rust is affecting a big part of the coil, the normal is to send it back to the supplier. Check transportation and

packaging responsibilities. It is believed that a big part of this problem is due to transportations and storages, but the research

in this area is still limited. If only a small part of the coil is affected by white rust, contact with your steel supplier to reach an

agreement if only a few blanks need to be scrapped.

Picture 7. Coils with white rust

Defects are visible at

several places in the coil

Usually >1/3 of coil width


5.1. Flat sheets stacked in packages

Stacks of blanks or sheets with water between layers will cause white rust as well. Treat laser cutted blanks same way as

above point.

6. How to avoid white rust

According to Gestamp studies [Martin Jnsson - Hkan Andersson, July 2014] and Arcelor reports [Arcelor, June 2014],

a method to greatly reduce white rust is to order the coils as micro oiled. Tests have shown that the coils form very little or no

white rust if coil is micro oiled. Long exposure to moist might still create white rust. See [Arcelor, June 2014]

7. Micro oiling

The principle of the micro oiling process consists of using a lubricating product in very small quantities. Our production and

R&D tests have been done using Arcelor material to approve the micro oiling process into our production lines. The blank/coils

have been oiled with two different oil brands used by the steel supplier: Quaker 6130 oil and Fuchs 4107. The range of oil

used is 0,5 0,3 g/m. All values and results showed into this guideline are based on this oil brands and oil quantities. If we

use a different type or oil quantity, a production test will be needed to validate that this oil is not affecting our furnace

atmosphere (dew point values).

The oil quantity is checked by controlling the flow of oil from the oiling machine at steel supplier. This is then checked and

compared with actual oil amount on the steel strip itself. Typically once to twice per shift must be checked by the steel

supplier. The approved oil quantities are 0,5 0,3 g/m in total (both sides).

The furnace atmosphere can be affected by the oil from the blanks. If dew point is rising, increase the amount of gas (dry air

and or N). Test shows that with enough amounts of gas and a stable furnace atmosphere, the dew point will not be affected

by the micro oiled blanks. See picture 8, see as well [Martin Jnsson - Hkan Andersson, July 2014]

Picture 8 - Dew point data tracking device

In this test, 150Nm/h of nitrogen is injected into a big line furnace. And as you can see on the graph, no differences along the

time were observed on the dew point values during the production of 3600 parts (approx. 10 hours continuous production).

Start production

with micro oiled

blanks

Finish production

with micro oiled

blanks


8. Quality control

If we are ordering coils directly from a steel supplier, oil film thickness (or g/m) must be added as a new parameter for the

quality checks. The control of oil amount on blanks/coils must be added into the incoming material quality control sheet. Later

this could be put as an annual parameter if the steel /blanks supplier has proven to deliver consistent and guaranteed quality.

There are a few methods for oil film thickness measurement that can be used:

Weighing a blank or test piece before and after cleaning. A very sensitive scale is needed (scale minimum up to 5000g,

accuracy +/- 0.05g). Clean blank with water and degreasing agent, then dry the blank before weighing. Calculate blank area

and then we have the g/m.

Oil film thickness measurement sensor (as example see: http://www.infralytic.de/). This kind of equipment is an accurate

device for quick measuring of the oil film thickness. The downside is that the equipment is expensive. We just recommend to

buy this device if youre consuming directly coils from the steel supplier (in house blanking operation).

If we are ordering the blanks from a blanking supplier, then an agreement with the blank supplier must be established about

who is responsible for the quality control, but they should have this equipment on their facilities.

9. References

[Arcelor, November 2012] - White rust defect on Usibor1500 Alusi - External - 11 2012

[Hkan Andersson, June 2013] - ED-paint adherence on press hardened parts made of Al-Si precoated boron steel with

white rust on the surface by Hakn Andersson (Gestamp Process R&D).

[Hkan Andersson, September 2013] - White staining products on the unhardened blank Effect on resistance spot

welding ability on the press hardened product by Hakn Andersson (Gestamp Process R&D).

[Azterlan, April 2013] - Usibor 1500P + AS150 parts / sheets white rust analysis. 19-2013.

[Arcelor, June 2014] - Temporary protection of -oiled Usibor1500 AS, Technical assistance to Gestamp, by H. Poli, E.

Jacqueson.

[Martin Jnsson - Hkan Andersson, July 2014] - Micro oiled coils as a temporary corrosion protection study


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White Rust Guideline - September, 2014