GB2335663A

GB2335663A - Galvanisation process with aluminium included for steel sheets

Info

Publication number: GB2335663A
Application number: GB9905795A
Authority: GB
Inventors: Thierry Schmitz
Original assignee: Electro Recherche SARL
Current assignee: Electro Recherche SARL
Priority date: 1998-03-26
Filing date: 1999-03-12
Publication date: 1999-09-29
Anticipated expiration: 2019-03-12
Also published as: FR2776672A1; IT1309081B1; ES2162725A1; SE9901116L; DE19913531A1; SE513833C2; GB2335663B; ITTO990232A1; GB9905795D0; SE9901116D0; ES2162725B2; TR199900625A2; TR199900625A3; FR2776672B1; BE1013517A3

Abstract

A galvanisation process for steel sheets, in particular assembled sheets such as structural parts for motor vehicles, distinguished by a particular silicon and phosphorus content, is characterised in that in a first pretreatment stage, the part to be treated is dipped in a pretreatment bath called a fluxing bath containing between 300 and 350 g/I of zinc chloride and between 100 and 150 g/I of ammonium chloride at a temperature in the order of 38C, so as to coat this part with an interface layer, and then the part thus coated is dipped in a galvanisation bath containing between 0.05% and 0.20% by weight of aluminium at a temperature of 440 to 450C, so as to obtain an even coating approximately between 10 and 40 Ám thick.

Description

2335663 1 GALVANISATION PROCESS FOR SHEET STEELS The present invention

concerns a process for galvanising steel sheets, particularly assembled sheets such as the structural parts of motor vehicles, distinguished by a particular silicon and phosphorus content.

It is well know that, in all areas of industry such as for example the automotive, household electrical appliance and canning industries...., it is necessary to protect iron, cast iron or steel parts against corrosion.

Among the processes most commonly used to etTect this protection one can mention galvanisation which has been known for several decades, This operation, which consists of covering the parts to be protected with a zincbased protective layer, can be carried out either by dipping in a bath of molten zinc (or roller coating), or by electrogalvanisation. The deposit, which must be of a controlled thickness, may or may not be applied to all sides of the part. It is composed either of pure zinc or zinc with added elements such as nickel, tin, copper, lead, iron, cobalt or aluminium.

It is to be noted in particular that the addition of aluminium improves the shine of galvanised coatings, reduces the surface oxidation of the zinc, improves the fluidity of the bath, and permits control of the zinc/iron reaction that helps in obtaining the required thickness.

To be satisfactory, galvanised coatings must have an even, non-mottled, shiny appearance and also have a uniform thickness as a general rule of the order of 10 to 40 pm.

Galvanisation baths containing aluminium can be grouped into a number of categories, viz:

- baths with a low aluminium content containing approximately 0.005% of aluminium, - baths with a medium content containing approximately 0.2% of aluminium, - so-called "polygalva" baths containing from d.04% to 0.05% of aluminium.

Although all non-alloy steels and malleable cast irons can be satisfactorily treated in a galvanisation bath containing aluminium, this is not the case for some alloy steels, in particular steels with high contents of silicon or phosphorus. In the case of these steels, the galvanisation operation can only provide coatings with a grey mottled matt finish c 2 which is unsatisfactory from an aesthetic point of view. Moreover, the presence in the steel to be treated of particular silicon and phosphorus contents tends to accelerate the growth of the zinc-iron alloy layers, which results in the coatiners obtained being thicker Z> and less resistant to impacts (risk of chipping under localised impacts).

It is to be noted in particular that, in the case of silicon steels, the thickness of the galvanised coating is not a linear function of the silicon content, as shown by the Sandelin curve in figure 1 which indicates how the thickness of a galvanised layer deposited on a steel part varies with the silicon content of this steel.

According to figure 1, one can distinguish two areas separated by a silicon content of 0. 1%. Steels with a silicon content below this limit are called "hyposandelin" while steels with a silicon content above this limit are called "hypersandelid'.

Figure 1 clearly shows that the growth of the galvanised layer is very rapid on approaching a silicon content of 0. 1 W, in this area, the layers formed are thick, fragile and grey.

To avoid the aforementioned problems standards have been set for steels intended for galvanisation, such as NF A 35-503. This standard defines two classes of steels by their silicon and phosphorus content.

Class 1.. Si:5 0.03% and Si + 2.5 P: 0.09% Class 2: Si:5 0.04% and Si + 2.5 P:5 0.11%.

These classes have been defined by statistical elements of distribution of the silicon and phosphorus contents.

It is generally accepted that steels with higher silicon and phosphorus contents than those mentioned cannot be galvanised satisfactorily, and even in the case of class 2 steels, if the contents are near the maximum limits, there can be a risk of obtaining a 25 thicker matt mottled coating.

It is to be noted that the industrialisation of the process called "poligalva" has, to a certain extent, reduced the disadvantages, without being completely satisfactory. Furthermore, it has not been possible so far to increase the aluminium content of galvanisation baths to percentages between 0.05% and 0.20%, in particular due to coating 30 adhesion problems encountered for these aluminium contents.

The present invention has the object of remedying this disadvantage by proposing a galvanisation process for steel sheets, in particular assembled sheets such as the ( ' 3 structural parts of motor vehicles, distinguished by a particular silicon and phosphorus content permitting an even coating to be obtained with a uniform, non-mottled shiny finish, which is generally satisfactory from the aesthetic point of view.

According to a preferred embodiment of the invention, this process is characterised in that in a pretreatment stage the part to be treated is dipped in a treatment bath called a "fluxing bath" containing between 300 and 350 g/1 of zinc chloride and between 100 and 150 g/1 of ammonium chloride at a temperate in the order of 38T so as to coat the part with an interface layer, then the part thus coated is dipped in a galvanisation bath containing between 0.05% and 0.20% by weight of aluminium at a temperature of the order of 440 to 450T.

The deposit, in accordance with the invention, of an interface layer between the surface of the part to be treated and the layer of galvanisation surprisingly provides a remedy for the problems linked with the use of "poligalva" baths with an aluminium content between 0.05% and 0.2%, and leads to even galvanised coatings being obtained approximately between 10 and 40 pim thick which are entirely satisfactory from the point of view of adhesion and aesthetics.

From experience it has been established that the part to be treated as a general rule must be immersed for approximately 30 seconds in the fluxing bath then for approximately 2 to 6 minutes in the galvanisation bath.

According to another characteristic of the invention, the pH of the fluxing bath is between 2 and 3 and is preferably of the order of 2.5.

In accordance with a preferred characteristic of the convention, it has been possible to galvanise satisfactorily parts of any type of steel and in particular of silicon or phosphorus steel by using a fluxing bath containing approximately 323 g/1 of zinc chloride and approximately 122 g/1 of ammonium chloride.

It has been possible to establish that such preliminary treatment leads to applying to the part to be treated an interface layer having the following composition:

40< Zn < 46% 54< NH4C1 < 60% 1/ 4 Experience has also shown that the galvanisation bath, preferably, should contain between 0. 090% and 0. 110% by weight of aluminium and also, if necessary, between 1. 0 and 1. 5% by weight of lead.

To verify the particularly advantageous character of the process according to the invention, it has been applied to the galvanisation of steel parts with high silicon and phosphorus contents.

Figure 2 shows the variations in thickness of the galvanisation coating thus obtained (in pim) in relation to the Si+2.5P content (as %) of these steels.

This figure clearly shows that, with the process according to the invention, coatings can be obtained of uniform thickness approximately between 10 and 40 PLm whatever the type of steel.

Claims

A galvanisation process for steel sheets, particularly assembled sheets such as structural parts of motor vehicles, distinguished by a particular silicon and phosphorus content, characterised in that in a first pretreatment stage, the part to be treated is dipped in a treatment bath containing between 300 and 350 g11 of zinc chloride and between 100 and 150 g/1 of ammonium chloride at a temperature in the order of 3 8T, so as to coat this part with an interface layer, then the part thus coated is immersed in a galvanisation bath containing between 0.5% and 0.20% by weight of aluminium at a temperature of 440 to OT, so as to obtain an even coating approximately between 10 and 40 Pirn thick.
2. A process according to claim 1, characterised in that the part to be treated is dipped in the treatment bath for approximately 30 seconds,
3. A process according to either of claims 1 and 2, characterised in that the pH of the treatment bath is between 2 and 3 and is preferably in the order of 2. 5.
4. A process according to any of claims 1 to 3, characterised in that the treatment bath contains approximately 323 gJ of zinc chloride and approximately 122 g/1 of ammonium chloride.
5. A process according to any of claims 1 to 4, characterised in that the part to be treated is coated with an interface layer having the following composition..

40< Zn < 46% 54< NH4C1 < 60%
6. A process according to any of claims 1 to 5, characterised in that the part to be treated is dipped in a galvanisation bath for approximately 2 to 6 minutes.
7. A process according to any of claims 1 to 6, characterised in that the galvanisation bath contains between 0. 090% and 0. 110% by weight of aluminium.
8. A process according to any of claims 1 to 7, characterised in that the galvanisation bath contains between 1. 0 and 1. 5% by weight of lead.
9. A galvanisation process for sheet steels containing silicon and phosphorus- the process comprising a first pretreatment step in which a part to be treated is dipped in a treatment bath containing zinc chloride (e.g. at a concentration between 300 and 350 g/1) and ammonium chloride (e.g. at a concentration between 100 and 150g/1) at a temperature in the range 33-43T (e.g. 35-40T, preferably approximately 38T) so as to coat the part - X \_ -,i 6 with an interface layer; and thereafter immersing the part thus coated in a galvanisation bath containing between 0.05% and 0.2% by weight of aluminium at an elevated temperature (e.g. in excess of 40TC, preferably 440-45OT) so as to obtain an even coating approximately 10 to 40 im thick.
10. A galvanisation process substantially as described herein with reference to the examples.