17 Improved Corrosion Protection through the Electrocoated Edge Spot-Weld Hem Design
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G E O R G E E. F.
BREWER
1
and
JAMES
W.
MITCHELL
F o r d Motor C o . , Manufacturing Development Center, 24500 Glendale A v e . , Detroit, M i c h . 48239.
It has been verified that slit-shaped rather than circular openings aid in the formation of electrodeposited films on inner surfaces of box-like structures.
An improved spot-
welded hem construction has been designed and developed recently, which creates slit-like gaps. The results on auto mobile doors are given as an example for the improved appearance,
greater mechanical
strength, and much im
proved corrosion protection resulting from the cooperation of designers, metal workers, and finishers.
' T ^ h e r a p i d , w o r l d w i d e a c c e p t a n c e of t h e electrocoating result of three m a i n advantages:
process is t h e
better c o r r o s i o n p r o t e c t i o n ,
cost, a n d v i r t u a l absence of p o l l u t i o n . T h e increased c o r r o s i o n
lower protec
t i o n is m a i n l y t h e result of t h e a b i l i t y to extend p a i n t films i n t o h i g h l y recessed areas, s u c h as b o x sections, channels, a n d other structures
with
entrance openings of s m a l l o p e n area, l e a d i n g to r e l a t i v e l y large i n n e r areas.
O b v i o u s l y , a larger entrance area w i l l p r o v i d e easier access f o r
the electric c u r r e n t a n d w i l l result i n a m o r e u n i f o r m p a i n t film d e p o s i t e d f r o m a n y given paint.
U s u a l l y , h o w e v e r , i t is n o t possible f o r t h e e n g i
neer a n d t h e stylist to p r o v i d e larger access holes since t h e m a t e r i a l strength a n d t h e a p p e a r a n c e of t h e structure m u s t b e m a i n t a i n e d . T h e influence of t h e size a n d shape of a n o p e n i n g o n the thickness of t h e e l e c t r o d e p o s i t e d p a i n t film, w h i c h is f o u n d i n s i d e of a c a v i t y , has been studied recently electrodepositable
(I).
T h e authors c o n c l u d e d that f o r a n y g i v e n
paint a n d structural configuration, the corrosion pro
t e c t i o n increases w i t h b o t h the o p e n area a n d the l e n g t h of t h e p e r i m e t e r of t h e o p e n i n g . L e t us consider a r o u n d hole of 0.5 i n c h i d . Its o p e n area 'Present address: 11065 East Grand River Rd., Brighton, Mich. 48116. 232 Brewer; Electrodeposition of Coatings Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
17.
BREWER
Edge Spot-Weld Hem Design
AND MITCHELL
233
is 0.20 s q i n c h , a n d its p e r i m e t e r is 1.8 inches. A r e a times p e r i m e t e r is t h e n 0.36 c u i n c h . N o w l e t us c o n s i d e r another o p e n area o f 0.20 s q i n c h , r e c t a n g u l a r i n shape, 1 i n c h l o n g b y 0.10 i n c h w i d e . Its p e r i m e t e r is 4.20 inches. A r e a times p e r i m e t e r is 0.84 c u i n c h , a n d a h i g h e r d e g r e e of c o r r o sion p r o t e c t i o n i s p r e d i c t e d f o r a structure w h i c h carries this t y p e o f opening.
T h e p r o b l e m was t h e selection o f a d e s i g n w h i c h w o u l d c o m
b i n e s l i t - l i k e openings w i t h m a t e r i a l strength, a n d o u r a t t e n t i o n w a s d r a w n t o w e l d e d joints. Downloaded by CHINESE UNIV OF HONG KONG on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0119.ch017
T w o sheet m e t a l parts, o r skins, w h i c h f o r m a t h r e e - d i m e n s i o n a l structure, s u c h as a n a u t o m o b i l e d o o r , are o f t e n d e s i g n e d t o b e h e l d together b y s p o t - w e l d e d h e m
flanges.
I n the conventional design of
these h e m flanges o n e s k i n is spot w e l d e d i n t o t h e 1 8 0 ° f o l d o f t h e other s k i n ( F i g u r e 1, u p p e r l e f t ) .
D u r i n g t h e operations o f c l o s i n g a n d spot
w e l d i n g there is a t e n d e n c y t o d i s t o r t t h e outer s k i n .
Experimentations
w i t h i m p r o v e d flange designs w e r e therefore i n d i c a t e d , p a r t i c u l a r l y t o take a d v a n t a g e o f e l e c t r o c o a t i n g a n d its a b i l i t y t o extend c o r r o s i o n p r o tective films t h r o u g h slit-like openings.
Edge Spot-Welded Hems T h e F o r d Manufacturing Development Center, i n conjunction w i t h the F o r d M e t a l S t a m p i n g D i v i s i o n , d e v e l o p e d a n e w h e m w e l d d e s i g n i n w h i c h o n e s k i n is b e n t a b o u t 1 3 5 ° t o meet a n a p p r o x i m a t e l y 9 0 ° b e n d of t h e outer s k i n ( F i g u r e 1, u p p e r r i g h t ) . T h e spot w e l d s create n a r r o w , slit-like openings b e t w e e n t h e edges o f t h e t w o skins ( F i g u r e 1, c e n t e r ) .
INNER SKIN
:
W
"
" ~ \ ^
A
^=^ CONVENTIONAL HEM FLANGE
\\EDGE WELD
= = = = = : = : : ;
^ 0 U T E R SKIN
NEW EDGE HEM F L A N G E
EDGE SPOT WELD
VIEW IN DIRECTION "A
M
Figure 1. Hemflangespot-welding comparison
Brewer; Electrodeposition of Coatings Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
234
ELECTRODEPOSITION
O F COATINGS
T h e s e slits a i d t h e e l e c t r o d e p o s i t i o n of c o r r o s i o n p r e v e n t i v e films o n t h e i n n e r surfaces of t h e structure. Automobile Doors. T h e strength o f t h e n e w 1 3 5 ° s p o t - w e l d e d h e m compares f a v o r a b l y w i t h that of t h e 1 8 0 ° h e m w e l d . w i t h adequate
Acceptable welds
strength c a n b e m a d e i n v a r y i n g flange heights.
The
a p p e a r a n c e o f t h e w e l d is g o o d , a n d d e s t r u c t i v e testing p u l l s " b u t t o n s " f r o m the s k i n .
T h e 1 3 5 ° w e l d d e s i g n leaves n o m a r k s o n t h e outside
metal
even
surface,
when
welds
are m a d e
under
widely
varying
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conditions. A n u m b e r of a u t o m o b i l e doors w e r e b u i l t b y u s i n g t h e n e w w e l d e d hems i n c e r t a i n areas ( F i g u r e 2 ) .
T h e s e doors w e r e p a i n t e d b y the
electrocoat m e t h o d , t h e n subjected to a v a r i e t y of tests w h i c h d e m o n strated m a r k e d i m p r o v e m e n t s i n c o r r o s i o n p r o t e c t i o n f o r i n n e r surfaces.
Figure 2.
Figure 3.
Door with edge spot-welded hem
Electrocoated doors after 250 hours of salt spray exposure
Corrosion Protection.
F i g u r e 3 compares t w o electrocoat
primed
a n d a c r y l i c e n a m e l e d doors after 250 hours of salt s p r a y exposure. T h e b o t t o m edge of o n e d o o r is f o r m e d b y a c o n v e n t i o n a l 1 8 0 ° h e m flange. It shows slight rust b l e e d i n g f r o m t h e i n s i d e a n d a l o n g the h e m edge. T h e other d o o r carries t h e n e w 1 3 5 ° h e m flange at its b o t t o m edge, a n d is free f r o m rust after 250 h o u r s of salt s p r a y exposure.
F i g u r e 4 shows
the test doors after t h e i r h e m flanges h a v e b e e n c u t o p e n .
T h e inner
surfaces of t h e 1 8 0 ° h e m s h o w slight c o r r o s i o n w h i l e t h e i n n e r surfaces
Brewer; Electrodeposition of Coatings Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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17.
BREWER AND MITCHELL
Figure 4.
Edge Spot-Weld Hem Design
235
Electrocoated doors after 250 hours of salt spray exposure—hem flanges cut open
of t h e n e w 1 3 5 ° h e m s h o w
a complete
paint
coat a n d absence o f
corrosion.
Summary T h e edge s p o t - w e l d e d h e m is c u r r e n t l y u s e d o n a v a r i e t y o f auto mobile components.
It improves the appearance
of t h e final p r o d u c t i n
several w a y s since i t is free f r o m d i s t o r t i o n o f t h e outer surface,
results
i n greater strength, constitutes a m i n o r m e t a l savings, a n d , i n c o m b i n a t i o n w i t h t h e e l e c t r o c o a t i n g process, results i n a h i g h e r degree of corro sion p r o t e c t i o n .
Literature Cited 1. Brewer, G. E. F., Wiedmayer, L. W., "Thickness of Electrocoats Inside of Cavities vs. Size and Shape of Openings," J. Paint Technol. (1970) 42 ( 5 5 0 ) , 588-591.
Brewer; Electrodeposition of Coatings Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
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ELECTRODEPOSITION OF
Brewer; Electrodeposition of Coatings Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
COATINGS
