The invention relates to a method for treatment of a surface area of steel by polishing said surface area and performing a plasma treatment of said surface area.

Such a method is known in the art and is applied to increase the adhesive bond durability of the steel particularly under harsh climatic conditions.

According to the invention a modification of this known method is proposed in order to further improve the adhesive bond durability of the steel and to attain further advantages as will become apparent from the following disclosure.

In a first aspect of the invention the plasma treatment that is carried out in the method of the invention is performed at at least atmospheric conditions. This provides the advantage firstly that the treatment can take place in the open air and that it may therefore be carried out in a simpler fashion and moreover cheaper than the known alternative of vacuum plasma treatment. A further advantage is that the method is applicable to steel surface areas having quite large dimensions.

Preferably the method of the invention is carried out with a plasma treatment occurring at a power of approximately 300 watt and for a duration of at least 20 minutes. A preferred exposure time is considered to be 30 minutes.

It may also be preferable that the plasma treatment is carried out at a pressure in the range of 3-5 bar.

In order to support and promote the effective plasma treatment according to the method of the invention it is desirable that the polishing to be carried out prior to the plasma treatment is a mechanical polishing so as to remove oxides from the steel surface area.

It may further be advantageous that the polishing is followed by ultrasonic cleaning for the removal of grease and/or dirt.

The invention will hereinafter be further elucidated with reference to the attached figures 1-5 wherein each figure shows comparatively the adhesive bond strengths of steel depending on a particular treatment or absence of treatment of said steel.

With reference first to Fig. 1 the influence is shown of applying atmospheric pressure plasma treatment and low pressure plasma treatment and its effect on the adhesive bond strengths of the steels so treated. The figure also shows a left bar representing the adhesive bond strength of steel which is untreated. What clearly follows from the figure is that an improvement of the adhesive bond strength of steel occurs when a low pressure plasma treatment is applied and that this bond strength may be further increased by applying an atmospheric plasma treatment of such steel.

With reference now to Fig. 2 a comparative example of the durability of the adhesive bond strength is shown of a steel that is exposed to atmospheric plasma treatment according to the invention. The figure shows said adhesive bond strength at respectively ambient conditions, low temperature conditions, elevated temperature conditions and under a thermal fatigue test. The adhesive bonds were separately exposed to said low temperature (-196°C), and said elevated temperature (+200°C) for approximately 100 hours, and the thermal fatigue test was carried out by exposing the joints of a steel bonded to another steel surface under 10 cycles of varying loads for a duration of two hours at the above-mentioned temperatures. It can be observed that the adhesive bond strength of the steel retains 96-98% of the bond strength that is present at ambient conditions.

Fig. 3, Fig. 4 and Fig. 5 are to be looked upon in combination. Fig. 3 relates to an atmospheric plasma exposed steel surface area, Fig. 4 relates to a low pressure plasma exposed steel surface area and Fig. 5 relates to an unmodified steel surface area.

Both Fig. 3, Fig. 4 and Fig. 5 show for the just-mentioned steels the adhesive bond strength at ambient condition and also when subjected to a high energy radiation with gamma and neutron radiation, and further when exposed to a Ringer's solution at an elevated temperature of 80°C for four weeks.

What transpires from figures 3, 4 and 5 is that without pre-treatment of steel as shown in Fig. 5 there is a comparatively strong deterioration of the adhesive bond strengths whereas the deterioration of the adhesive bond strengths is less severe when the steel is subjected to a low-pressure plasma treatment as shown in Fig. 4. Best results are however obtained when the steel surface area is exposed to an atmospheric plasma treatment which is clearly shown in Fig. 3.

It can therefore be concluded that the pre-treatment of steel by atmospheric pressure plasma treatment is highly beneficial to attain a durable and strong adhesive bond.