GB2092123A

GB2092123A - A Method of Applying a Resistant Protective Enamel Coat to Ductile Cast Iron Objects

Info

Publication number: GB2092123A
Application number: GB8136178A
Authority: GB
Original assignee: Eisenwerke Fried Wilh Dueker GmbH and Co KGaA
Current assignee: Dueker GmbH
Priority date: 1980-12-01
Filing date: 1981-12-01
Publication date: 1982-08-11
Also published as: GB2092123B; DE3045249A1; FR2495190B1; FR2495190A1; JPS57149844A; SE8107160L; SE445038B

Abstract

The invention relates to a method of applying a protective enamel coat which is resistant to aggressive media, to ductile cast iron objects, in particular cast iron pipes, comprising applying an enamel to the cast iron, object surface which does not comprise a ground enamel coat and has been pre-treated in well-known manner, which enamel has the following enamel composition matrix: <IMAGE> In this manner it is surprisingly possible to apply the enamel directly-i.e. without previous application of a ground enamel.

Description

SPECIFICATION A Method of Applying A Resistant Protective Enamel Coat to Ductile Cast Iron Objects The invention relates to a method of applying a protective enamel coat which is resistant to aggressive media, to ductile cast iron objects, in particular cast iron pipes.

As described in literature ("Silikat-Technik" 1 955, page 480) one was of the opinion from the first that ductile iron ("Sphdrogup"-nodular iron) could not be enamelled. At about the same time it became known (Petzold "Emaille" 1950, Stuckert "Die Emailfabrikation" 1941 and Marker "Emailliertechnik" 1956) that it is necessary to first of all apply and burn-in a ground enamel before resistant enamel powders are laid on. Accordingly this implies a two-stage proceeding. Such a proceeding entails a high amount of electrical energy and great expense owing to the ground enamel coat which has to be applied and burnedin at first.

The invention is based on the object of further developing the conventional method so that also ductile cast iron objects can be provided with a resistant enamel coat by one-stage enamelling operation.

The method according to the present invention by which this object is achieved, is substantially characterized in that an enamel is applied to the cast iron object surface which does not comprise a ground enamel coat and which has been pretreated in a well-known manner, which enamel has the following enamel composition matrix:: % in weight SiO2 30-50 B203 2-10 Na20 15-20 Bi203 1-10 Li2O 1-5 TiO2 5-15 CaO 1-5 BaO 1 Awl203 1-3 ZrO2 1-3 CeO2 0-3 SrO 0--1 ZnO 1-5 MnO O3 NiO 0.5-2 CoO 0.5-1 F2 0.5-2 The teaching of the present invention therefore is based on the surprising perception that-unlike the aforementioned prevailing belief-it is possible to apply to ductile cast iron objects, in particular cast iron pipes, and by means of a direct-acting method i.e. without previous application of a ground enamel, an enamel coat resistant to organic acids and to any kind of water, so that for burying such cast iron pipes in the ground one need not provide a special envelope or previously arrange an additional protective layer to protect them against aggressive media. Apart from the possibility to save energy as the costly application of a ground enamel coat which must be burned-in a special step can be done without, the use of spheroidal graphite or nodular cast iron pipes provided with a protective enamel coat means substantial simplification and thus causes great reduction in labour and time.

The enamel is applied preferably in perpendicular position of the cast iron pipes or of like rotational-cast objects, viz. in particular in the form of a slip by means of dipping, spraying or rinsing. For drying the applied slip most conveniently an induction coil can be used. The enamelling proper, i.e. the firing of the enamel, is carried out while the cast iron objects take the vertical position, preferably also by means of an induction coil at preset temperature and speed.

The pipe is passed through the induction coil from top to bottom, or vice versa.

With the method according to the present invention it proved most practical to apply the enamel in the form of powder directly to the cast iron object while the latter is still in the heated state. Such a proceeding is most advantageous in that time may be saved. Besides, as the enamel used according to the invention has a relatively low meltdown temperature so as to fulfil the condition that first of all the cast iron object to be enamelled can be heated to the temperature at which the enamel will adhere and then can be enamelled while the still existing inherent heat of the cast iron object is being used, a separate heating and firing operation may practically be omitted and additonally a high amount of energy can be saved.

With the afore-mentioned low meltdown temperature there is connected a low softening interval of 500 to 5200C, by means of which the enamel may be melted smooth at a temperature range of from 700 to 8000C. By reducing the burn-in time, e.g. to 1 to 3 minutes, one may proceed also in the upper temperature range of up to 9500 C.

The enamel used in the method according to the subject invention is characterized by its low surface tension of 200 to 210 dyn/cm. This allows quick wetting and flat-flowing of the enamel in the thin-layer range of less than 400 microns. In the thin-layer range of less than 500 microns the enamel is impact resistant and wear resistant up to 5 kp. Owing to the dry technology a non-adulterate enamel glass powder is melted, so that compact and smooth surface enamel coats are obtained which by no means tend to incrustation. Due to the intimate bond of the bonded material-enamel and ductile cast ironcorrosion or rust formation beneath the surface will not occur. The enamel glass powder material is neutral with respect two all metals and metal compounds. It should also be noted that the enamel is harmless in terms of physiology and toxicity, on the one hand, and resistant in the pH range of from 1 to 10 to all organic acids and waste waters as well as raw water, service water and drink water, on-the other hand.

Claims

1. A method of applying a protective enamel coat which is resistant to aggressive media, to ductile cast iron objects, in particular cast iron pipes, characterized in that an enamel is applied to the cast iron object surface, which does not comprise a ground enamel coat, which enamel has the following enamel composition matrix: % in weight SiO2 30-50 8203 2-10 Na2O 15-20 Bi203 1-10 Li2O 1-5 TiO2 5-15 CaO 1-5 BaO 1-5 Al2O3 1-3 ZrO2 1-3 CeO2 0-3 SrO 0-1 ZnO 1-5 MnO 0-3 NiO 0.5-

2 CoO 0.5-1 F2 0.5-2 2. A method according to claim 1, characterized in that the enamel is applied, in the form of powder, directly to the cast iron object which is still in the heated state.

3. A method according to claim 1, characterized by the use of an induction coil for drying the enamel slip applied to said cast iron object and/or for firing the protective enamel coat.

4. A cast iron object enameled according to the method of one of the preceding claixns.