SELECTIVE CONVERSION COATINGS ON NICKEL AND STAINLESS STEEL

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SELECTIVE CONVERSION COATINGS ON NICKEL AND STAINLESS STEEL

J. Mason, R. Blower

To cite this version:

J. Mason, R. Blower. SELECTIVE CONVERSION COATINGS ON NICKEL AND STAINLESS STEEL. Journal de Physique Colloques, 1981, 42 (C1), pp.C1-231-C1-246. �10.1051/jphyscol:1981117�.

�jpa-00220667�

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JOUIZNAL DE PHYSIQUE

CoZZoque C1, suppZ6ment au nO1, Tome 42, janvier 1981 puge C1-231

SELECTIVE CONVERSION COATINGS ON NICKEL AND STAINLESS STEEL

J.J. M a s o n and R. B l o w e r (r)

Inco E u r o p e L t d . , B i r m i n g h a m , U. K., ( a ) MPD T e c h n o l o g y L t d .

1. I n t r o d u c t i o n . - Chemical conversion p r o c e s s e s o f f e r p o t e n t i a l f o r t h e production of low-cost s e l e c t i v e s u r f a c e s a t hiqh production r a t e s . Two such s u r f a c e s have been developed by Inco Europe Ltd.

The f i r s t , a chemically grown oxide f i l m on s t a i n l e s s s t e e l , / 1 - 4 / , was developed over t h e p e r i o d 1966 t o 1972 f o r d e c o r a t i v e f i n i - s h e s , and a s such i s a v a i l a b l e i n a ranue of c o l o u r s . Although t h e re- f l e c t a n c e s p e c t r a of s e v e r a l o f t h e c o l o u r s were given i n an e a r l y pa- p e r / 2 / and t h e s e demonstrated q u i t e good s o l a r absorptance, combined w i t h high r e f l e c t a n c e i n t o t h e near i n f r a r e d , t h e widesgread i n t e r e s t

i n t h e t r e a t m e n t f o r s o l a r c o l l e c t o r s d i d n o t develop u n t i l t h e work of Smith /5/ and o t h e r s /6/ i n A u s t r a l i a , and more r e c e n t l y Karlsson /7/.

Blue c o l o u r e d s t a i n l e s s s t e e l , b l u e having t h e b e s t s o l a r ab- s o r p t a n c e o f t h e range o f c o l o u r s , has an absorptance v a l u e 0.90 combined with a 100° hemispherical e m i t t a n c e of 0.13.

The second s u r f a c e , produced by a p r o p r i e t a r y p r o c e s s on n i c k e l /8-lo/, i s e s s e n t i a l l y a t h i n mixed n i c k e l oxide. I t h a s enhanced s o l a r absorptance (0.97) a s s o c i a t e d w i t h a t o p o l o g i c a l microroughness, combined with low thermal e m i t t a n c e E h l O O o C = 0.09. Both s u r f a c e s a r e cur- r e n t l y produced on a commercial s c a l e by PlPD Technology Ltd. The s t a i n - l e s s s t e e l s u r f a c e t r e a t m e n t , marketed under t h e name Skysorb ( ' ) is a v a i l a b l e f o r a l l grades of s t a i n l e s s s t e e l i n s i z e s up t o 2m x l m . Fi- n i s h e d components can be coloured o r , a l t e r n a t i v e l y , s h e e t s of coloured s t e e l can be s u p p l i e d f o r subsequent in-house f a b r i c a t i o n of components.

E s s e n t i a l l y , t h e process i s a b a t c h process. The n i c k e l s u r f a c e , termed Piaxorb ( I ) , i s a v a i l a b l e i n two forms : Maxorb f o i l and Maxorb p l a t e .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981117

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The Maxorb f o i l i s produced i n t h i c k n e s s ranging from 1 0 t o 50 um. The s t a n d a r d t h i c k n e s s f o i l , 13 pm, i s s u p p l i e d adhesive backed f o r a p p l i - c a t i o n t o m e t a l l i c and non-metallic s u b s t r a t e s . Some o f t h e advantages of t h e concept of f o i l s f o r t h e a p p l i c a t i o n o f s e l e c t i v e s u r f a c e s were p o i n t e d o u t by Mason /9, 10/ and more r e c e n t l y reviewed by Lampert /11/.

C u r r e n t l y , Maxorb i s produced i n widths up t o 148mm, b u t l a t e r t h i s y e a r w i l l be a v a i l a b l e up t o 500mm. laaxorb p l a t e has t o d a t e been ac- p l i e d t o components up t o 3m i n l e n g t h .

I n t h i s paper, we s h a l l review t h e p r o p e r t i e s of both t h e s e se- l e c t i v e s u r f a c e s and dwell on t h e l a t e n t r e s u l t s , p a r t i c u l a r l y t h o s e r e l a t i n g t o a s p e c t s of d u r a b i l i t y .

2 . P r o p e r t i e s o f Maxorb.- 2.1. O p t i c a l ropert ties and s t r u c t u r e . - A s d e s c r i b e d e a r l i e r , t h e s o l a r absorptance of Paaxorb i s very high, being measured a t 0.97 f o r AH2 spectrum. The r e p e a t a b i l i t y o f t h e process i s such t h a t t h e s o l a r absorptance can be kept w i t h i n t h e range 0.96 t o 0.98 d u r i n g a production run of s e v e r a l thousand square metres.

We had p r e v i o u s l y r e p o r t e d / 9 / t h e thermal e m i t t a n c e of t h e s u r - f a c e a t 100°C a s 0.08 t o 0.12 depending upon t h e method of measurement.

This d i f f e r e n c e h a s been r e s o l v e d i n t h e work of P e t t i t /12/ who has shown t h a t t h e v a l u e s given by t h e Willey emissometer have t o be c o r r e c t e d t o c o r r e l a t e with o t h e r measurement methods.

Table 1.- T o t a l hemispherical e m i t t a l ~ c e o f Maxorb s o l a r f o i l

Table I l i s t s v a l u e s of t o t a l hemispherical e m i t t a n c e f o r Maxorb f o i l measured c a l o r i m e t r i c a l l y by W.W. Beens e t a l . /13/. Also included a r e

Temperature OC

29.3 47.8 97.4 120.7 151.4 100 100

( I ) Trade Mark MPD Technology Ltd.

Hemispherical Emissivity

€h

0.065 0.071 0.079 0.088 0.098 0.08 0.07

Measurement Method

Calorimetric

Corrected Willey Emission Gier Dunkle Emissometer

Source

Beens 13

*

Mason 9

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c o r r e c t e d Willey r e s u l t s and v a l u e s o b t a i n e d w i t h a Gier Dunkle emissometer.Agreement i s very good.

2.2. Durabil_i_ty o f t h e s u r f a c e . - Maxorb h a s been s u b j e c t e d t o a wide v a r i e t y of t e s t s t o e s t a b l i s h i t s r e s i s t a n c e t o d e g r a d a t i o n under c o n d i t i o n s p r e v a l e n t t o f l a t p l a t e c o l l e c t o r s and under c o n d i t i o n s which a r e considered t o a c c e l e r a t e t h e d e g r a d a t i o n processes. Some of

t h e t e s t s have been performed by Masters /14/ a s p a r t of a much wider programme, aimed a t t h e development of s t a n d a r d s f o r e v a l u a t i n g s o l a r absorber m a t e r i a l s . Some t e s t s were performed in-house.

The s u r f a c e o p t i c a l p r o p e r t i e s , measured a f t e r ageing i n a i r f o r v a r i o u s p e r i o d s of time up t o 300°C, a r e l i s t e d i n t a b l e 11. There i s no s i g n i f i c a n t change i n p r o p e r t i e s over t h e c o n d i t i o n s examined.

Table 11.- E f f e c t o r a i r ageing on t h e o p t i c a l p r o p e r t i e s of Maxorb

(+) Masters /14/

Time/Days

0 10 84 98 200 463

Heat exposure f o r s h o r t e r p e r i o d s of time, up t o 1 hour, suggest t h a t t h e s u r f a c e i s s t a b l e i n a i r up t o 400°C /9/. C l e a r l y , t h e s u r f a - c e h a s adequate temperature s t a b i l i t y f o r use i n s i n g l e o r double g l a - zed c o l l e c t o r s .

Moisture r e s i s t a n c e was determined u s i n g both c y c l i c ( t o M I L STD 810B) and s t e a d y s t a t e t e s t i n g . I n view of t h e a c c e l e r a t e d n a t u r e of t h e s e t e s t s , t h e r e was s u r p r i s i n g l y l i t t l e change i n absorptance ( s e e Table 111).

Although t h e r e s u l t s i n d i c a t e a high degree o f s t a b i l i t y , t h e i r s i g n i f i c a n c e can only be a s s e s s e d i n r e l a t i o n t o t h e equivalence b e t - ween such t e s t s and r e a l time exposure. Unfortunately, s i n c e Maxorb has only been produced f o r 3 y e a r s

-

no long time r e s u l t s a r e a v a i l a b l e .

AGEING TEMPERATURE

150°C 2OO0C 250°C

as

0.98(+)

0.97(+)

I

3OO0C a~

0.98

"s

0.98 0.98(+:

0.97 0.98 'h100

0.09

0.08

's

0.99 'hl00

0.08

---..---..---..---

' 'hl00

0.09 0.10 0.07 0.09

'h100

0.09

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Table 111.- E f f e c t o f m o i s t u r e on s o l a r p r o p e r t i e s o f Maxorb s o l a r f o i l

( + ) R e s u l t s due t o M a s t e r s / 1 4 /

I n o u r t e s t s , samples have shown no measurable change i n s u r f a c e p r o p e r t i e s a f t e r s t a g n a t i n g i n a s i n g l e - g l a z e d i n s u l a t e d box on o u r l a - b o r a t o r y r o o f f o r 2 y e a r s . S i m i l a r s t a g n a t i o n t e s t s , t o d a t e f o r a s h o r - t e r p e r i o d , have been r e p o r t e d by M a s t e r s /14/ and a r e reproduced i n t a b l e I V . These t e s t s a r e c o n t i n u i n q .

T a b l e 1V.- S t a g n a t i o n t e s t s a t t e s t s i t e s i n t h e U.S.A.

S o l a r P r o p e r t i e s

2 . 3 . S t a b i l i t y - o f - a d h e s i v e . - Our p r e v i o u s work / 9 , 10/ d e s c r i b e d how a s i l i c o n e - b a s e d p r e s s u r e s e n s i t i v e a d h e s i v e had been e l e c t e d f o r f i x i n g t h e f o i l t o s u b s t r a t e s b e c a u s e o f i t s combination of t e m p e r a t u r e s t a b i - l i t y and w e a t h e r i n g r e s i s t a n c e . I n o r d e r t o a s s e s s t h e d e g r e e t o which t h e f o i l i s bonded t o t h e s u b s t r a t e , p e e l t e s t s (Method ASTM D1000)

Surface

Maxorb F o i l on ~1

I n i t i a l values

as h' 100

0.98 0.09

Test s i t e

NBS

253 days a 'hi00

0.98 0.09

Arizona

128 days

Eh 0.98 0.09

Miami

124 days

as Eh

0.98 0.08

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were performed.

The r e s u l t s o f t e m p e r a t u r e s up t o 2 0 0 ° C o n t h e p e e l s t r e n g t h o f & I a x o r b o n A l a r e i l l u s t r a t e d i n f i g u r e l . These r e s u l t s a r e f o r a 90° p e e l mode.

The performance o f DC 282 (produced by Dow Corning) t h e a d h e s i v e i n i t i a l - l y s e l e c t e d f o r flaxorb f o i l , shows a f a l l o f f i n p r o p e r t i e s a f t e r an i n i t i a l c u r e o n t o t h e s u b s t r a t e . I n s p i t e o f t h i s , t h e r e was no s i g n o f edge d e l a m i n a t i o n even a f t e r t h e most s e v e r e c o n d i t i o n s (200 d a y s a t 2OO0C).

More r e c e n t l y , a s i l i c o n e w i t h h e a t s t a b i l i z a t i o n S R 6574 (General E l e c t r i c S i l i c o n e s ) h a s been e v a l u a t e d . The r e s u l t s ( s e e Fig. 1) show a s i g n i f i c a n t improvement o v e r DC282.

L ^I

0 1 10 100 moo

EXPOSURE days

F i g . 1.- E f f e c t o f t e m p e r a t u r e on t h e a d h e s i o n o f Maxorb t o aluminium f o r two a d h e s i v e s ^DC 232 and S R 6574.

The above r e s u l t s r e l a t e t o aluminium a s a s u b s t r a t e . S i m i l a r t e s t s have been made f o r copper and m i l d s t e e l s u b s t r a t e s .

The b e h a v i o u r w i t h m i l d s t e e l i s v i r t u a l l y i d e n t i c a l t o aluminium I n t h e c a s e o f c o p p e r , however, t h e r e i s e v i d e n c e o f d e l a m i n a t i o n a f t e r e x p o s u r e a t 200°C. T h i s a p p e a r s t o b e due t o o x i d a t i o n o f t h e copper under t h e a d h e s i v e which c l e a r l y commences a t t h e edge o f t h e f o i l .

The f o i l a l s o a c t s a s a h u m i d i t y b a r r i e r and p r o t e c t s t h e under- l y i n g s u b s t r a t e . T h i s i s p a r t i c u l a r l y e v i d e n t upon e x a m i n a t i o n o f Maxom f o i l c o a t e d A 1 and m i l d s t e e l samples a f t e r c y c l i c h u m i d i t y t e s t s .

On removal o f t h e f o i l , a c l e a r c u t - o f f can be s e e n between t h e

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p r o t e c t e d and unprotected a r e a s . Some i n d i c a t i o n o f t h e e f f e c t of cy- c l i c humidity temperature t e s t s on t h e adhesion o f t h e two adhesives examined can be seen from f i g u r e 2 . Under t h e c o n d i t i o n s o f MIL S T D 8 1 0 8 temperatures up t o 71°C continued with high h u m i d i t i e s (95% R . H . ) , the- r e i s no s i g n i f i c a n t change i n adhesion a f t e r 100 days. These t e s t s a r e c o n t i n u i n g .

MIL 8108 DC 282

c

O o 1 10 1 0 0

CYCLES ( D A Y S )

Fig. 2.- E f f e c t o f humidity c y c l i n g on t h e adhesion of Maxorb t o aluminium

2.4. Adhesion t o ngn-metals.- A s w e l l a s i t s a p p l i c a t i o n t o metals i n f l a t - p l a t e s o l a r c o l l e c t o r water h e a t e r s , Maxorb s o l a r f o i l may be con- s i d e r e d f o r use i n p a s s i v e systems. A r e c e n t t h e o r e t i c a l a n a l y s i s by J.S. Van Wieringen /15/ showed t h a t double g l a z i n g i s n e c e s s a r y with n e u t r a l black-painted w a l l s i n o r d e r t o g e t reasonable o u t p u t . However, a s i n g l e glazed w a l l with a s e l e c t i v e s u r f a c e g i v e s i n c r e a s e d y i e l d , b e cause of b e t t e r e f f i c i e n c y and a r e d u c t i o n i n n o c t u r n a l l o s s e s , and holds o u t t h e promise of lower c o s t .

P r e l i m i n a r y experiments, aimed a t a s s e s s i n g t h e adhesion o f lfaxorb, with i t s normal P.S.A. adhesive backing (DC 282) t o common c o n s t r u c t i o n m a t e r i a l s such a s b r i c k and c o n c r e t e showed t h a t t h e r e was l i t t l e adhe- s i o n , due t o t h e uneven and/or f r i a b l e n a t u r e of t h e s e s u r f a c e s . I n o r - d e r t o s t a b i l i s e t h e s u r f a c e , v a r i o u s p r e - t r e a t m e n t s were examined.

The most s u c c e s s f u l t r e a t m e n t , one meeting t h e requirement o f t e r n p e r a t u r e s t a b i l i t y t o g e t h e r w i t h room temperature c u r e i n a i r , was a s i l i c o n e r e s i n produced by Dow Corning d e s i g n a t e d R-4-3117 Conformal C o a t i n g . P e e l s t r e n g t h s a f t e r up t o 10 days a t l5O0C a r e given i n t a - b l e V.

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T a b l e V.- Adhesion o f Maxorb (DC282) t o common b u i l d i n a m a t e r i a l s

C o n c r e t e , h a v i n g an i n h e r e n t a b s o r b e n t i r r e q u l a r s u r f a c e , c a l l e d f o r t h e a p p l i c a t i o n o f s u c c e s s i v e l a y e r s o f m a t e r i a l s h a v i n g p r o g r e s s i - v e l y smoother s u r f a c e s . These m u l t i l a y e r specimens, w i t h a R-4-3117 f i - n i s h r e a l i z e d t h e h i g h e s t p e e l s t r e n g t h s , t y p i c a l l y 380g/cm.

S u r f a c e s c o a t e d w i t h a s t a n d a r d f i n i s h i n ? p l a s t e r i . e . T h i s t l e F i n i s h , s u f f e r e d from s p a l l i n g a f t e r exposure t o 150°C. S i n c e t h i s was used f o r t h e a c c e l e r a t e d t e s t s t h e p l a s t e r was r e p l a c e d by T e t r i o n f i l - l e r (manufactured by T e t r o s y l L t d . ) which proved s t a b l e a t t h i s tempe- r a t u r e .

B r i c k , having a lower a b s o r p t i v i t y t o l i q u i d s and g e n e r a l l y a smoother s u r f a c e t h a n c o n c r e t e responded t o s i m p l e s u r f a c i n g r o u t e s . R e s u l t s from a s i n g l e l a y e r o f R-4-3117 a p p l i e d d i r e c t l y t o t h e b r i c k s u r f a c e were o n l y a b o u t 20 % lower t h a n t h o s e o b t a i n e d from a t h r e e l a - y e r t r e a t m e n t o f c o n c r e t e . The r e s u l t s s u p p o r t s t h e p r e m i s e t h a t Plaxorb can be a p p l i e d f a i r l y r e a d i l y t o b r i c k o r c o n c r e t e s u r f a c e s .

2.5. S u r f a c e p r o t e c t i o n . - L i k e a l l h i g h a b s o r p t a n c e s e l e c t i v e s u r f a c e s Maxorb i s p r o n e t o f i n g e r marking. I t i s n o t p o s s i b l e t o remove t h e mark s u b s e q u e n t l y . Although t h e e f f e c t on s u r f a c e p r o p e r t i e s i s minimal, changing t h e a b s o r p t i v i t y from 0.98 t o 0.97 and h a v i n g no measurable e f f e c t on e m i t t a n c e , such marking i s u n s i g h t l y .

Attempts t o improve h a n d l e a b i l i t y f o c u s s e d on o v e r l a y i n g t h e b l a d c s u r f a c e w i t h a t h i n p l a s t i c l a y e r . A Dow Corning s i l i c o n e r e s i n , DC 808, d i s p l a y s a s u i t a b l e combination of p r o p e r t i e s , namely, good h e a t r e s i s - t a n c e combined w i t h good f l e x i b i l i t y i n t h e c u r e d c o n d i t i o n . A t h r e e r o l l r e v e r s e c o a t i n g t e c h n i q u e on a commercial r o l l e r c o a t e d combined w i t h d i l u t i o n o f t h e r e s i n w i t h up t o 90 % s o l v e n t e n a b l e d t h e a p p l i c a -

t i o n o f c o n t r o l l e d t h i c k n e s s e s down t o a b o u t 0.1 pm.

The e f f e c t o f r e s i n t h i c k n e s s on o p t i c a l p r o p e r t i e s i s i l l u s t r a - Peel Strength

g/cm

178 337 380 380 178 315 315

I

Substrate

Concrete

Brick

I,

Coatings

1 2 3

Cement Tetrion R-4-3 117 render

I II

R-4-3117

- -

Pre-test Exposure

A s applied 24h/15OoC 4 days 15OoC 10 days 150eC A s applied 24h/150°C 4 days 150°C

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c1-238 JOURNAL DE PHYSIQUE

t e d i n f i q u r e 3 . F o r u s e i n f l a t p l a t e c o l l e c t o r s an e m i t t a n c e o f 0.15, c o r r e s p o n d i n g t o a r e s i n f i l m t h i c k n e s s o f 0.5 um, would b e q u i t e sa- t i s f a c t o r y . T h i s s u r f a c e l a y e r would p r o v i d e added s e c u r i t y a g a i n s t pos- s i b l e d e g r a d a t i o n , t h e o n l y d i s a d v a n t a g e b e i n g t h e reduced s o l a r a b s o r p - t i v i t y ( 0 . 9 3 5 ) .

0.90 0 0 U) 2.0 30 0

.RESIN THICKNESS p m

Fig. 3.- O p t i c a l p r o p e r t i e s o f llaxorb w i t h DC 808 f r o n t c o a t i n g

2.6. g a x o r b p l a t i n g . - There a r e t h r e e main a r e a s where Maxorb f o i l may n o t be u s e d , i n c o n c e n t r a t i n g c o l l e c t o r s where t h e h i q h e r t e m p e r a t u r e s p r e v e n t t h e u s e o f a d h e s i v e s , i n vacuum t u b e s where d e g a s s i n g o f t h e a d h e s i v e can c a u s e problems, and i n f l a t p l a t e c o l l e c t o r s h a v i n g pro- f i l e d s u r f a c e s u n s u i t a b l e f o r l a y i n g f o i l . I n a l l t h r e e c a s e s t h e component can be n i c k e l p l a t e d and t h e n blackened u s i n g t h e same p r o c e s s a s used f o r f o i l . I n g e n e r a l , t h e s o l a r p r o p e r t i e s o f t h e blackened p l a t e a r e v e r y s i m i l a r t o t h e f o i l , t y p i c a l v a l u e s b e i n g a = 0.96,

E hlOOOC = 0.12.

3 . Coloured s t a i n l e s s s t e e l . - The I n c o c o l o u r i n g p r o c e s s produces an o x i d e f i l m on t h e s u r f a c e of t h e s t e e l which i s b a s i c a l l y c o l o u r l e s s and t r a n s p a r e n t . A s t h e f i l m - t h i c k n e s s i n c r e a s e s a r a n g e o f i n t e r f e r e n - c e c o l o u r s a r e o b t a i n e d i n t h e o r d e r b r o n z e , b l u e , g o l d , p u r p l e and g r e e n / 3 / .

A s formed t h e s e f i l m s a r e s o f t and porous. An e l e c t r o l y t i c t r e a t - ment i s used t o s e a l t h e f i l m s . T h i s t r e a t m e n t c o n f e r s wear and s c r a t c h r e s i s t a n c e on t h e f i l m s and a l s o improves t h e i r h a n d l e a b i l i t y , p e r m i t - t i n g them t o b e d e g r e a s e d and c l e a n s e d w i t h o u t d e t r i m e n t t o t h e i r appea- r a n c e .

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3.1. Clp&~ca~-pggpegg&gg.- These f i l m s have been shown t o be s o l a r se- l e c t i v e / 2 , 5/.

Values of s o l a r absorptance o b t a i n e d on i n t e r f e r e n c e f i l m s u s i n g r e l a t i v e l y simple l a b o r a t o r y t e c h n i q u e s , such a s t h o s e developed f o r a s s e s s i n g black s u r f a c e s , a r e of d o u b t f u l v a l i d i t y . The reasons f o r t h i s have been d i s c u s s e d by Mason /16/.

Values of s o l a r absorptance of 0.90

-

0.95 have been o b t a i n e d i n t h e Skysorb b l u e range of c o l o u r s u s i n g t h e s e t e c h n i q u e s , which i s i n agreement w i t h f i g u r e s quoted g e n e r a l l y by o t h e r s working on s i m i l a r f i l m s . However, i t i s f e l t t h a t t h e s e can be misleading and t h e i r use should be discouraged.

A n a l t e r n a t i v e method of e s t i m a t i n g s o l a r absorptance and one which i s perhaps o f q r e a t e r p r a c t i c a l s i g n i f i c a n c e , i s by use o f e f f i - c i e n c y curves. Using an ASHRAE 93-77 s t a n d a r d procedure t h e e f f i c i e n c y curves were determined f o r two i d e n t i c a l c o l l e c t o r s f a b r i c a t e d from A.I.S.I. Type 304 annealed s t a i n l e s s s t e e l , one c o a t e d with Skysorb

s e l e c t i v e s u r f a c e , t h e o t h e r p a i n t e d matt b l a c k . (Nextel 101-C10 Black Velvet)

-

^Skyscrb ^surface

.-- - -

. Nextel = d a c e

Fig. 4.- Performance graph of two s t a i n l e s s s t e e l c o l l e c t o r s

The performance curves o b t a i n e d a r e shown i n f i g u r e 4 . The e f f i - c i e n c i e s of t h e c o l l e c t o r s a r e expressed a s

Skysorb s u r f a c e q = 0.823

-

4.65

-

AT

a

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JOURNAL DE PHYSIQUE

Nextel surface

n

= 0.869

-

^6.86

-

AT

_Q

The ratio of the two maximum efficiency values is the ratio of the respective solar absorptances.

Taking ci Nextel as 0.97 tkis gives a Skysorb as 0.92. This is in rsasona- ble agreement with values calculated for similar films by Smith /5/

from reflectance spectra using the A.M.2 solar spectrum.

The thermal emittance at 100°C, EhlO0, measured using a Willey emissometer did not vary over the range of blue colours obtained by the Inco process. Thicker films i-e., purple and green, gave total hemispherical emissivities at 100°C of 0.12

-

^0.15.

Auger spectra were taken on two samples ~~A.I.s.I. Type 304 steel representing the bronze/blue border and blue/gold border, that is either end of the blue range obtained by the Inco process. These were sputtered to give semi-quantitative chemical analysis of elements as a function of depth.

Table V1.- Auger electron spectroscope. Depth profile analysis of coloured stainless steel, Type 304.

Sample

Bronze/Blue

~lue/Gold

Approx Thickness wm 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.10 0.14 0 0.01 0.03 0.05 0.07 0.09 0.11 0.14

COMPOSITION W t %

S C 0 Cr Fe Ni

6.7 2.8 36.0 26.9 19.6 7.6

0.9 0.3 28.4 23.8 32.0 13.8

0.8 0.3 24.7 22.5 36.7 14.4

0.6 0.2 18.3 17.9 48.4 14.0

0.5 0.2 13.1 58.7 13.3

0.3 0.2 8.0 69.5 12.4

0.2 0.2 5.8 73.6 11.8

0.0 0.2 3.0 77.8 10.8

0.0 0.3 0.0 80.7 9.5

0.0 0.3 0.0 79.6 9.5

7.3 1.6 39.1 27.3 17.7 6.3

1.1 0.2 36.2 30.6 19.8 11.4

1.2 0.0 33.0 29.6 22.2 13.4

0.9 0.0 30.2 26.2 26.5 15.5

0.7 0.1 22.1 19.3 41.7 15.7

0.4 0.2 10.7 62.8 12.9

0.1 0.3 2.4 76.5 10.3

0.0 0.4 0.0 79.3 9.5

I

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The r e s u l t s , t a b l e V I , i n d i c a t e t h a t t h e oxide i s r i c h i n chro- m i u m compared w i t h t h e s u b s t r a t e s t e e l , b u t a l s o c o n t a i n s i r o n and n i - c k e l . The oxide t h i c k n e s s over t h i s c o l o u r range v a r i e s from approxima- t e l y 0.09 ym t o 0.13 um.

3.2. Durabiligy of Skyggmb-surfaced s t a i n l e s s s t . 3.2.1. Atmospheric c o r r o s i o n t e s t s . - Corrosion t e s t s have been made a t a number o f indus- t r i a l and urban s i t e s on Skysorb s u r f a c e d s t a i n l e s s s t e e l s o v e r p e r i o d s extending t o 13 y e a r s f o r samples without t h e hardening t r e a t m e n t and e i g h t y e a r s f o r samples with t h e hardening treatment. Generally, samples were exposed on south f a c i n g r a c k s a t 45' t o t h e h o r i z o n t a l and were unprotected. No a t t e m p t was made t o s t i m u l a t e f l a t p l a t e c o l l e c t o r t e s t environment.

There was no evidence of c o l o u r change i n any of t h e specimen a m - pared with unexposed c o n t r o l s . A.I.S.I. Type 304 and Type 430 s t e e l s were exposed. The c o r r o s i o n r e s i s t a n c e of t h e coloured and hardened s m - p l e s of both t y p e s of s t e e l was g e n e r a l l y s u p e r i o r t o t h a t of u n t r e a t e d samples.

Skysorb s u r f a c e d A.I.S.I. Type 304 s t e e l samples have a l s o been exposed a t Narine t e s t s i t e s . P a n e l s f a c i n g seawards were placed 250 m from t h e ocean. A l l samples were unprotected.

Table V I 1 . - Exposure of Skysorb s u r f a c e d s t a i n l e s s s t e e l t o marine a t - mospheres

A.S.T.M. Rating

Material

Skysorb surface on Type 304 L

A.I.S.I. 304 L A.I.S.I. 304 L

10 Na defective area 9 0 . 1 % defective area 8 0.25% defective area

1

A. S.T.M. Corrosion Rating 2

.

⁵ ^years^Exposure

9 9 8 9 6 5

7 0.5% defective area 6 1.0% defective area 5 2.5% defective area

Colour r e t e n t i o n a f t e r 2 . 5 y e a r s ' exposure was very good. Only a very s l i g h t d i f f e r e n c e was d e t e c t e d when comparing exposed p a n e l s w i t h unexposed c o n t r o l s .

A . S . T.M. c o r r o s i o n r a t i n g d a t a on t h e marine atmosphere c o r r o s i o n t e s t samples

i s

cjiven i n t a b l e V I I . Skysorb s u r f a c e d samples showed only minimum c o r r o s i o n s p o t t i n c j and were s u p e r i o r t o u n t r e a t e d samples.

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JOURNAL DE PHYSIQUE

3.2.2. A c c e l e r a t e d c o r r o s i o n t e s t s . - The g e n e r a l t r e n d s o f t h e atmos- p h e r i c c o r r o s i o n tests were reproduced i n a c c e l e r a t e d t e s t i n g c o n d i t i o n s

CASS T e s t s were made on A.I.S.I. Type 304 and Type 430 s t e e l s w i t h and w i t h o u t Skysorb s u r f a c e s a c c o r d i n g t o p r o c e d u r e s s p e c i f i e d i n BS4601, 1970. A f t e r 24h e x p o s u r e , t h e Skysorb s u r f a c e d p a n e l s showed l e s s r u s t s t a i n t h a n t h e u n t r e a t e d p a n e l s . Blue c o l o u r e d b u t unhardened p a n e l s were i n f e r i o r t o Skysorb s u r f a c e d p a n e l s .

C y c l i c t e s t s i n an atmosphere c o n t a i n i n g 500ppm o f s u l p h u r d i o x i - de were made on Skysorb s u r f a c e d Type 304 s t e e l . The m o i s t u r e c o n t e n t o f t h e atmosphere was g r e a t e r t h a n 80% and t e s t t e m p e r a t u r e s v a r i e d i n t h e r a n g e 24-48OC. A f t e r 200 c y c l e s , e a c h o f one h o u r d u r a t i o n , t h e r e was no change o f appearance o f t h e Skysorb s u r f a c e d p a n e l s . U n t r e a t e d A.I.S.I. Type 304 s t e e l showed some c o r r o s i o n .

R e s u l t s o f p i t t i n g c o r r o s h o n tests i n a s o l u t i o n o f 10% f e r r i c c h l o r i d e f o r 1 h o u r a t 20°c a r e p r e s e n t e d i n t a b l e V I I I .

T a b l e V I I 1 . - C o r r o s i o n t e s t s on Type 304 s t a i n l e s s s t e e l s i n 10% f e r r i c c h l o r i d e s o l u t i o n a t room t e m p e r a t u r e f o r one h o u r .

The Skysorb s u r f a c e d s t e e l showed s u p e r i o r r e s i s t a n c e t o a t t a c k t h a n t h e c o l o u r e d b u t unhardened o r t h e u n t r e a t e d s t e e l .

3.2.3. S t a g n a t i o n t e s t s . - Skysorb c o a t e d samples o f s t a i n l e s s s t e e l A.I.S.I. Type 304 w i t h b o t h m i r r o r p o l i s h e d and b r i g h t a n n e a l e d s u r f a - c e s and A . I . S . I . Type 444 s t e e l m i r r o r p o l i s h e d were exposed i n s i n g l e g l a z e d , b a c k - i n s u l a t e d boxes on o u r Birmingham L a b o r a t o r y r o o f . The v e n t e d boxes a l l o w e d c o n d e n s a t i o n and t h e n a t u r a l change i n a i r . Com- p a r i s o n w i t h unexposed c o n t r o l samples showed no change i n c o l o u r and no e v i d e n c e of r u s t s p o t t i n g a f t e r 1 . 5 y e a r s exposure. These t e s t s a r e c o n t i n u i n g .

L

Specimen Condition

Unco l o u r e d

Blue unhardened Skysorb hardened

Uncoloured

Skysorb (Hardened)

P i t damage

~ / c m * 14.0

7.0

< 1.0

35.0

5.0

. Remarks

L a r g e & f i n e p i t s

Large deep p i t s One o r two i s o l a t e d s m a l l p i t s Large & f i n e p i t s

Very f i n e &

shallow p i t s S u r f a c e

Finish

Mirror P o l i s h

2B

Appearance

Moderate P i t t i n g Moderate/heavy p i t t i n g Very s l i g h t p i t t i n g

V e r y heavy p i t t i n g Very s l i g h t p i t t i n g

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3 . 2 . 4 . Thermal s t a b i l i t y . - T e s t s have been made t o determine t h e e f f e c t of h e a t on t h e c o l o u r s t a b i l i t y , adhesion and appearance of t h e colou- r e d f i l m .

Exposure of t h e coloured and hardened s t e e l t o b o i l i n g water f o r 20 days showed no change i n any of t h e s e p r o p e r t i e s .

Exposure i n a d r y atmosphere a t 2 0 0 ' ~ f o r 20 days, showed a s l i g h t a l t e r a t i o n i n shade d u r i n g t h e e a r l y s t a g e s of exposure. I t i s thought t h a t t h i s i s due t o t h e removal of water p r e s e n t i n t h e f i l m .

I n d i c a t i o n s of c o l o u r change f o r l o n g e r exposure t e s t s a r e l i s t e d i n t a b l e IX.

Table 1X.- E f f e c t of long term thermal exposure on c o l o u r s t a b i l i t y of s t a i n l e s s s t e e l s .

The c o l o u r changes f o r t h e s e l o n g e r times a r e d i c t a t e d by t h e growth r a t e of t h e oxide f i l m . This p r o c e s s depends upon t h e i n t r i n s i c oxida- t i o n r e s i s t a n c e of t h e s u b s t r a t e s t e e l a s w e l l a s t h e temperature and time of exposure, Of t h e t h r e e t y p e s of s t e e l examined, t h e oxide s t a - b i l i t y of t h e simple f e r r i t i c s t e e l ( 1 7 % C r ) i s lowest, t h e a u s t e n i t i c s t a i n l e s s s t e e l , (Type 304) ranks n e x t , w h i l s t Type 4 4 4 ( c o n t a i n i n g 18Cr 2Mo) h a s t h e g r e a t e s t r e s i s t a n c e t o oxide growth a t t h e t e s t tem- p e r a t u r e s used. Because of t h e f l a t maximum i n s o l a r absorptance, over t h e whole range of b l u e c o l o u r s , t h e c o l o u r changes produced by up t o

112 days a t 150°C f o r Types 304 and 4 4 4 s t e e l s have l i t t l e e f f e c t on s o l a r absorptance. The change i n appearance, and t h e r e f o r e , r e d u c t i o n i n s o l a r absorptance f o r t h e simple 1 7 % C r . S t e e l a f t e r 112 days a t 150°C probably p r e c l u d e s i t a s a m a t e r i a l f o r f l a t p l a t e s o l a r c o l l e c - t o r s

.

The f i l m growth p r o c e s s i s m o r e r a p i d a t 200°C. A l l t h e s t e e l s tes- t e d showed some c o l o u r i n s t a b i l i t y a t t h i s t e s t temperature.

V a r i a t i o n i n s o l a r absorptance f o r m i r r o r p o l i s h e d b l u e s t a i n l e s s s t e e l A.I.S.I. Type 304 exposed f o r l h a t temperatures up t o 500°C a r e shown i n f i g u r e 5. S o l a r absorptance was c a l c u l a t e d from t o t a l r e f l e c -

S t e e 1 Type

3 0 4 4 4 4 1 7 C r 3 0 4 4 4 4 1 7 C r

Temperature

150°C 15OoC 1 5 0 OC 2OO0C 2OO0C 200°C

A s coloured

& hardened

Blue Blue Blue Blue Blue Blue

Period o f Exposure -

a f t e r 5 6 days

Blue somewhat paler Blue

Light blue Lighter blue Blue

Blue/Gold

a f t e r 1 1 2 days

Blue Paler blue Blue/Gold Blue/Gold Very l i g h t blue Gold

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t a n c e s p e c t r a . Mirror p o l i s h e d samples were chosen because they o f f e r e d a more uniform s t a r t i n g s u r f a c e . These i n d i c a t e d t h a t s h o r t time, i . e . , minutes, exposure a t temperatures up t o 300°C i s n o t d e t r i m e n t a l t o t h e

s o l a r p r o p e r t i e s o f t h e s u r f a c e .

AlSl Type 304 stainless steel

Fig. 5.- One hour temperature exposure on absorptance of b l u e s t a i n l e s s s t e e l

The r e s u l t s o f thermal s t a b i l i t y t e s t s i n d i c a t e t h a t s k y s o r b coa- t e d A.I.S.I. Type 304 and Type 444 have s a t i s f a c t o r y thermal s t a b i l i - t y f o r s i n g l e g l a z e d f l a t p l a t e s o l a r c o l l e c t o r s . I t i s l i k e l y t h a t Ty- pe 316 a u s t e n i t i c s t e e l would have b e t t e r c o l o u r r e t e n t i o n t h a n Type 304 and probably be s i m i l a r t o Type 4 4 4 .

Although t h i s a s p e c t of f i l m growth p r e c l u d e s use of Skysorb s u r - f a c e s f o r s o l a r c o l l e c t o r s l i k e l y t o experience s e r v i c e c o n d i t i o n s i n - volving long p e r i o d s of exposure i n a i r a t temperatures above 200°C ie. concentratinqcollectors, i t does n o t preclude t h e i r use f o r such

c o l l e c t o r s where t h e s u r f a c e i s p r o t e c t e d by vacuum.

3.3. Fgpabil_i_ty.- The adherence o f t h e oxide f i l m has been a s s e s s e d by s u b j e c t i n g Skysorb c o a t e d s t e e l s t o v a r i o u s commercial c o l d forming processes.

Coloured s h e e t s up t o 1200mm by 900mm have been r o l l p a t t e r n e d using a p r o c e s s t h a t r e s u l t s i n no o v e r a l l change i n dimensions o f t h e s h e e t and produces l o c a l r e d u c t i o n s i n t h i c k n e s s o f up t o 30%. S i m i l a r s h e e t s have been deformed u s i n g r o l l forming and conventional p r e s s i n g techniques.

I n none of t h e s e t e s t s was t h e r e any evidence t h a t t h e f i l m had cracked o r become detached from t h e underlying metal. Samples have been b e n t through s h a r p a n g l e s and undergone l a b o r a t o r y deep drawing t e s t s w i t h s i m i l a r r e s u l t s .

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A clear advantage of this surface over other reflective surfaces is, therefore, its suitability to a wide range of cold forming methods after colouring.

4. Conclusion.- This paper has reviewed the salient features of two selective surfaces, one on nickel ; the other on stainle~ssteels~which are both now commercially available. Both surfaces have differin? advantages which make them complementary rather than competitive in the market place. Skysorb surfaced stainless steel, which has even better atmospheric corrosion resistance than conventional stainless steel, is ideally suited to lower temperature applications. In its gresent form, it is not recommended for use in non-evacuated concentrated collectors.

The surface has excellent formability and handleability allowing the fabricator plenty of scope in maximising its design potential. This colouring process is a batch process.

The Maxorb surface on nickel is a high performance selective absorber which either as a foil or plated component finish can be applied to all substrates. As indicated previously, the blackening can either be a batch or continuous process. In its continuous form, it allows a greater degree of control and Greater flexibility in product form. The surface does not have as good atmospheric corrosion resistance as coloured stainless steel but possesses better thermal stability. The cost of both processes is very similar.

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References

/1/ Evans,T.E., Hart,A.C., James,H., Smith,V.A., Trans. Inst. Met. Fin.

50

^{(1972) 77}

/2/ Evans,T.E., Hart,A.C., Skedgell,A.N., Trans.

Inst. Met. Fin.

51

(1973) 108

/3/ Blower,R., Evans,T.E., Sheet Metal Industries 51, No.5 (1974)

-

/4/ Hart,A.C., UK Patent 1, 305 636

/5/ Smith,G.B., flet. Australasia (1977) 204 /6/ Granziera,F., Met. Australasia (1977) 211 /7/ Karlsson,B. and Ribbing,C.G., S.P.I.E.,

Optics applied to solar e n e r g

161

(1978) 76-83

/8/ Wriqht,B., Mason,J.J., Proc. I.S.E.S. Meeting New Delhi, India (1978) 1080

/9/ Mason,J.J., V7right1B., Proc. 2nd Int. Solar Forum, Hamburg, Germany (1978) 337-350 /lo/ l~ason,J.J., While,L.G., To be published. Pre-

sented at Comples Int. Solar Enerqy Conf., Nilan (1979)

/11/ Lampert,C.H., 2nd Symposium Coatinqs for Solar Collectors, St. Louis, MO (1979)

/12/ Pettit,R.B., J. of Enaineering for Power Trans. ASEIE (1978) 1-8

/13/ Beens,W.W., Sikkens,M., VerSter,J.L., J. Phys E, 13 (1980) 873-876

-

/14/ Masters,L.W., Waksman,D., 2nd Symposium Coatings for Solar Collectors, St Louis, MO (1979)

/15/ Wieringen,J.S., Private communication

/16/ Whittle,J.E. and Mason,J.J., Paper to Ajaccio, Conference (1980)