phosphate coatings for deep drawing of steel tubes ... substrate in a special zinc phosphate solution. ... Equilibrium shifts to the right, and insoluble tertiary.
COATINGS HglI#OLO! ELSEVIER
Surface and Coatings Technology 102 (i998) 233-236
The influence of hexametaphosphate on formation of zinc phosphate coatings for deep drawing of steel tubes V. Burokas *, A. Martugien4, G. Bikul~ius Institute o/" Chemistry, Vilnius, Lithuania Received I7 July 1997; accepted 12 December 1997
Abstract The fact that application of the phosphate coatings facilitates cold formation of metals is we11known. Depending on the type of cold forming (extrusion, drawing, stamping), various phosphating solutions are used. The deep drawing of tubes with a reduced wail thickness, when the drawing length is up to 50% that of the initial length, appears to be of great difficulty. Special phosphate coatings are used for this purpose. The combination with sodium stearate or other lubricants allows an item to be formed in one deep drawing step, without damaging the equipment. This paper deals with the influence of hexametaphosphate (HMF) on the weight (thickness), roughness of phosphate coatings and interaction with sodium stearate (NS). © 1998 Elsevier Science S.A.
Keywords: Deep drawing; Hexametaphosphate; Lubricant; Zinc phosphating
1. Introduction The use of zinc phosphate coatings to facilitate the cold formation of steel is well known [1,2]. Extrusion of steel with zinc phosphate coatings impregnated with various lubricants is a subject of many original and patent studies [3-6]. Phosphate coatings are formed after immersing a steel substrate in a special zinc phosphate solution. In such solutions, certain equilibria are established [7]: 3Zn 2+ + 2HzPO4 ~Zn3(PO4)2~ + 4 H +
( 1)
H a P O 4 ~ H + + HzPO2 •
(2)
H + ions are consumed when metal dissolution takes place: M e + 2 H + --+Mea+ + H z T
(3)
Equilibrium shifts to the right, and insoluble tertiary zinc phosphate is deposited on the metal surface. This process is especially accelerated when oxidants (NO;or other ions) are added to the solution. In this case, the following reaction proceeds [8]: 4 M e + 10H + + N O ; - o 4 M e z+ + N H 2 + 3 H 2 0 .
determines which ions discharge first. Besides the essential coat-forming components (Zn 2+, P O ] - ions), some additives that regulate the structure of the deposits (crystal size and compactness) and maintain a constant acidity ( p H ) etc., are added to the solution [9]. Phosphate coatings function as an NS absorbing agent, which permits a greater reduction of the tube wall thickness in one drawing step, prevents the direct contact of the equipment with the tubes and improves corrosion resistance. The introduction of H M F contributes to the formation of such phosphate coatings, which allows tubes to be drawn in one step up to 50% from the initial length. The aim of this study is to investigate in detail the influence of H M F on phosphate coating formation and NS absorbtion.
(4)
The ratio of H + and NO5 concentrations directly a Corresponding, author.
0257-8972/98/$19.00 © 1998 ElsevierScience B.V. All rights reserved. PII S0257-8972(98)00359-4
2. Experimental For phosphating, steel samples with an area of 60 cm 2 (C=0.085-0.900%, S=0.004%, P = 0 . 0 2 % ) were used. Before phosphating, the samples were prepared in a standard hot alkaline degreasing solution (Na3PO4, 30-40 g dm-3; NaOH, 40-50 g dm-3; N a 2 C Q ; 30-40 g dm -3, at T = 7 0 - 8 0 °C for 5-10 rain), and after rinsing, they were acid-pickled (HC1 1:1) and
234
V. Burokas et aL / Surface and Coatings Technology102 (t998) 233-236 The pH value of precipitation of the insoluble tertiary zinc phosphate "PIP" (point of incipient precipitation) was established by titration of 100 ml of solution with 0.2 N N a O H (indicator phenolphtaleine). The value of ApH was calculated from: ApH = pHem -
$
I I
r i-=
/4
,I
I
Fig. I. Principal scheme of practical deep drawing of the tubes with press type 210. carefully rinsed again in running water. The samples were phosphated without special any activation. The chemical composition of the standard bath is as follows: Zn 2+ - 0 . 1 7 M; P O ] - - 0 . 3 4 M; Ni 2+ - 0 . 0 1 7 M; NO;- - 0 . 1 4 M. Different amounts of H M F was used, varying from 0.01 to 0 . 5 0 g d m -a. Phosphating was carried out by immersion at 55 °C temperature, p H = 1.8 -2.4, immersion time ( z ) = 0 . 0 - 1 5 m i n . The substrates were lubricated by means of immersion in a 70 g dm -3 solution at 55 °C for 5 min. All chemicals used in the preparation of the solutions were of reagent grade. The press type 210 (24060 Bagnatica--BE Italy) was used for the sliding tests. The principal scheme of drawing and some parameters are shown in Fig. 1 and Table 1. The weight of phosphate coating (Pf), weight of absorbed NS (Ps) and mass of dissolved metal were calculated from weighing data obtained with the help of an analytical balance ( A D V - - 2 0 0 M, Russia). The accuracy of weighing was _+0.0001 g. The substrates were weighed after surface degreasing, phosphate-coating deposition, surface lubricating, dissolution of the lubricant in hot water (at 90 °C, z = 5 min and after removal of the phosphate coating in 25% solution of CrO3 (at 70 °C, ~=5 rain). The data shown in the figures and tables are averages of five to 10 weighing tests.
pHsolution.
The pH value in the "thin layer" of solution was determined in a special cell with a capacity of 2 cm 3 into which the 60 cm 2 area electrode was immersed [10]. The pH value was determined in 15 s after phosphating. The corrosion resistance was investigated using the " d r o p " method [11]. The composition of the solution was: 40 ml 0.5 M CuSO4, 20 ml 10% NaC1 and 0.8 ml 0.1 M HC1. The solution was dropped on to the test area of the surface and the time taken for the bluish colour of the solution to turn brown was recorded. The corrosion resistance was deemed satisfactory (coating weight is normal) when none of the drops on the surface became completely brown in 30 s. The size of the crystals was investigated using a microscope (ORIM-1, Russia). The crystal size data as represented in the tables were measured at five different points on the surface. The roughness of the phosphate surface was estimated by means of the prophilometer--prophilograph type 252 (Russia). The prophylograms of the coating surface were drawn, and the roughness parameters (R~) were measured [12].
3. R e s u l t s and discussion
3.1. The influence of H M F on physicaI properties of phosphate coatings The weight of the coatings in the standard phosphate solutions was regulated by oxidants and special additives (hydroxylamine, tripolyphosphate, tartrate, etc.) [3-6]. The phosphate coatings obtained were too thin or their crystalline structure was too fine, and therefore, the thickness is not sufficient for deep drawing. The drawing of the substrates using a press showed that on the surface with a coating weight greater than 1 2 g m -2 (heavy coating), the so-called "wave" was formed, the resistance of which was so high that the tube broke. In the opposite case, when the weight of
Table 1 Substrate characteristics Substrate number
1 2
External diameter (~, mm)
Length of substrate (H, mm)
Before drawing
A~er drawing
Before drawing
ARer drawing
30.0 28.0
28.6 25.4
220 200
275 300
V. Burokas ez al. / Surface and Coatings Technology 102 (1998) 2 3 3 - 2 3 6
C•vi ~,"~ 40
235
02 t =-2
5
0
I
I
!
5
40
4~
Fig. 2. The influence of hexametaphosphate (HMF) on the coating weight and the quantity of absorbed natrium stearate (NS): 1 and 2, coating weight (PO; I' and 2', coating weight after lubricating (Pf+Ps). 1 and 1', without HMF; 2 and 2', with HMF, 0.2 g dm -3.
CL
},os
! i
2mm Fig. 4. Prophilogrammes of the phosphate coatings obtained at various concentrations HMF (gdm-3): (a) 0.0i, (b) 0.I0, (c) 0.20, (d) 0.50. Standard solution at 55 °C, ~= I0 min.
I
I
2. mm Fig. 3. Prophilogrammes of phosphate coatings obtained at various time (~): (a) 0.0, (b) 5, (c) 10 and I5 rain. Standard solution without HMF, T=55 '~C.
the coatings was less t h a n 8 g m -2 (thin coatings), d a m a g e to the tools was observed. T h e use o f H M F in p h o s p h a t i n g s o l u t i o n leads to the f o r m a t i o n o f a so-called " f l a k e " structure. In this case, a sufficient q u a n t i t y o f N S was a b s o r b e d , a n d only a slight resistance o f the slide was observed. The f o r m a t i o n o f p h o s p h a t e coatings was c o m p l e t e d in 10 m i n (Figs. 2 a n d 3). T h e weight d a t a i n d i c a t e d t h a t the thickness o f the coatings c o u l d be r e g u l a t e d b y c h a n g i n g the H M F c o n c e n t r a t i o n ( T a b l e 2). T h e structure o f the p h o s p h a t e c o a t i n g was c h a r a c terized by roughness f a c t o r (Ra). This c h a n g e d f r o m 4.36 to 0.57 ~m ( T a b l e 2). P h o s p h a t e coatings with the
Table 2 The influence of hexametaphosphate (HMF) on the properties of phosphate coatings (standard solution, T= 55 °C, z = 10 min) Number
1 2 3 4
HMF (g dm -3)
coating (ff, g m--')
Weight of coating after lubricating (Pf+ Ps, g m- 2)
Weight of absorbed sodium stearate (P~, g m-2)
Size of crystals (lam)
Parameter of roughness (Ra, l-Lm)
0.01 0.10 0.20 0.50
37.92 11.86 7.85 4.20
41.88 15.96 11.18 7.23
3.96 4. I0 3.33 3.03
30-40 20-25 20-25 5-10
4.36
Weight of
1.73
1.31 0.57
V.. Burokaser aL / Surjace and Coatings Technology102 (1998) 233-236
236
Table 3 The influence of pH on the weight of phosphate coating (Pf) and mass of dissolved metal in standard solution with 0.2gl -~ HMF (T=55 °C. ~= i0 min) SolutionpH
DepositionpH
ApH
Pf(gm -2)
Pm(gm -2)
1.8 2.0 2.2 2.4
4.0 3.8 3.4 3.2
2.2 1.8 1.2 0.8
15,2 I3.8 11.4 11.2
8.2 7.5 5.7 5.3
highest value R~ were too thick and did not have a developed surface. Meanwhile, the coatings with minimal Ra values had minimal coating weights and were too thin for the drawing purposes. This effect is in agreement with the corrosion test. The coatings deposited from solution 4 (Table 2) did not pass the 30-s corrosion test. Practical drawing tests using the press showed that the best sliding results were with coatings deposited from the solution with 0,1-0.2 g d m -3 ( H M F ) (Table 2), The weight of absorbed NS (Table 2) correlated with the surface development [Fig. 4(b and c)].
3.2. The influence phosphating sohttion acidio, on the physicat properties of the coatings According to the mechanism of the phosphate-coating formation (Eqs. (1)-(4)), the acidity ( p H ) of the solution has a strong influence on the process. The presence of oxidants (NO;- ions) has no influence on the p H value but accelerates the formation of the phosphate coatings. The p H value of the solution should be as close as possible to the pH value of "PIP". Measurements of p H values by the "thin layer" method allow the pH value at which insoluble phosphate deposition occurs to be determined. Tests were made in a standard solution with 0.2 g dm -3 H M F at a temperature of 55 °C and a deposition time of 10rain. These data are shown in Table 3. It was found that when the acidity of the solution
was below 1.6 (pH < 1,6), deposition of phosphate coatings occurred on separate sites and did not cover all the surface. When the pH was above 2.8, insoluble tertiary phosphates were deposited in the whole of the bath. Table 3 shows that the values of p H solution and pH deposition are inversely related. A minimal zXpH value was reached when the pH of solution was 2.2-2.4. The drawing tests confirmed that substrates plated in the solution with these pH values proceeded without any complications.
4. Conclusions This investigation indicated that deep drawing of the tubes ( ~ 50% length) with a reduction in wall thickness necessitates phosphate coatings with specific characteristics (weight, structure and roughness). It was determined that the optimal range of H M F is 0.1-0.2 g dm-3. In this case, the weight of the phosphate coating varies from 8.0 to 12.0 g m -2 and the factor of roughness (Ra) varies from 1.3 to 1.7 ~m, correspondingly. H M F additive has an optimal effect at p H values in the range of 2.2-2.4.
References [1l W. Rausch, Die Phosphatierung yon Metallen, Eugen G. Leuze, Saulgau, Germany, I988. [2] D. James, Sheet Metal Industr. 3 (1961) 171. [3] H. Bialoctocka, H. Kolankowa, E. Maslankiewicz, Powloki ochronne 8990 (1988) 45. [4] USA Patent, No. 45117029 (1985). [5] German Patent, No. 4306446 (1993). [6] British Patent, No. 2169620 (I986). [7] V. t2upr, H. Pleva, Metalloberflache25 (1971) 89. [8] W. Jaenicke, B. Lorenz, Werkstoffeand Korrosion 10 (I959) 681. [9] G. Lorin, Phosphating of Metals, Fin, Publ. Ltd, London, 1974. [10] V. Burokas, A. Martugiene, Chemija 3 (1996) I4. [ll]I.I. Khain, Teorija i Praktika Phosphatirovanija Metatov, Khimija, Leningrad, 1973, [12] GOST 2789-73. Surface Roughness. Parameters and Characteristics (Russian standard).
