Surface Technoh;gy. ~,. ( ~ e "7 ) . . , , I ~-~v,, - I I " , © E l s e v i e r S e q u o i a S . A . , t~ausanne - - Pr]~ ~ed in t h e Nether~;~ n.d~

THE F O R M A T I O N A N D P E R F O R M A N C E O F C H R O M A T E CO N V ERo ' "' ~ ~ I N ( ; * SION C O A ~ , I N ~ S ( )N Z

L. F. G. W I L L I A M S

Australian Defe~.~ce Sci,:ntific Service, Moierials Research Labor~t(:,rie~, P.O. Bo:x 50, Ascot Vah', Vic. 302::, /~!etbournc ( A u ~ r a ~ )

( R e c e i v e d S e p t e m b e r 2, 1 9~ 6 )

S u m m a r y

This paper dea!,s wi tb ~he m e c h a n i s m s (,f f o r m a t i o n ( f chror, qate ,:o~- vers ion films on zinc and thei r p e r f o r m a n c e characte~4¢/i-'.~, The c omp~,,dt~[)~ o f c h r o m a t e c o n v e r s i o n films is rov iewed t o g e t h e r wit h the ef fec ts o f an ions such as su lpha te .

Ini t ia l ly , aging o f fi lms .esmt,s in t h e we l t -known ha rden ing o f t he coat- ing, b u t s u b ~ q u e n t l y , o t h e r changes can occur , suci~ as c racking o f t h e fi lm and a r e d u c t i o n in t h e de~orp t ion ra te o f tLe c L r o m a t e f rom tbe film. Tiie co r ros ion p r o t e c t i o n c o n f e ~ e d by tlae fi!n:s is (iel~e,.~oe,~t: or~ ~ bar~]e'~ n a t u r e o f the p r ec ip i t a tod c t~romiumII I I ) h y d r o x i d e .a,~'*_.._; on . . . . the) inbi}~i~. :...~oo.,,,~, p rope r t i e s of t he a d s o r b e d c h r o m a t e , With i r :~e~ ( ~ ~.~: ~- ~}~0' ~,,'-~:~ . . . . . . . . . , . ,

of t he c h r o m a t e and , t.h,a~,:'efore, tl~e 1)rotec~,~oa give;, by t!=,. f~'-n~ arc afro., t ed by the aging proceeds.

~.ccelerated test n~e~..hods, s~ch ,~:, ~n(~ salt s p r a ) * shown to be unreliah~,, The c h r o ' n a I c conte~)t of th~ • fi!m ¢,.)~,s ~:()t ,~r~...~,,~ the co r ros ion r~sis tance, p r o b a b l y b,~,cause {a) the qual_,ty of the ur~der ly~~ zinc plaice has a subs t an t i a l e f fec t on ~he co r ros ion reac t ion a ~ ~b} the leacI~ ing rate or d e s o r p t i o n ~f the c h r o m a t e is dependvn~ on aai~,,,, ~ °}~ films.

Int]~c,ductmn

C h r o m a t . . . . . . . . . . ~ ~ ! ~ . ~ v ~ ~ ~ ! ~ ~ ~ ~

h u m i d s torage c o n d i t i o n s and ~t ~ i'te,a . : ~ r o n ~ a t e d ~vh(w. ti ~,s i,o be sto~°cd for e x t e n d ~ per iods , In recen~ years ¢h~,~e t~a~ been a~ i~o~vas~¢g !r¢~d t~.)v~.~r~i l~he use o f zinc e ! e c t r o p i a t e b e c a u ~ • o f ~he t ox i c i t y oi cadres, urn a~~d ~s

. . . . ~ L ~ , | k ~ t ~ ' : ' ~ " ~ ~ ~ i ; ~}~,~' ~'P~w~;r p r e p a r e d Zo~ ~ ~ . . . . . ~ ~ F i n i s h i n g , , O ~ o b e r .~ 9 ~ ~.

106

TABLE 1

Classification of ch roma te eoaversion coat ings

Coal3ng tyoe Typica~ appearance Typical coat ing weight ( g m 2)

Typica l Cr tVt) con t en t (rag m 2)

A-(~iear

B-Bleached

C-Iridescent

D-Opaque

"l~ansparent, clear some tbnes ~.~ ith at bluish tinge

Transparent wi th slight ir idescence

Yel low iridescent

O![ive green shading to b rown or b ronze

- - - - . - ~ - , ~ .

up to 0.5 5

up to 1,0 5 - ~0

1.0 - 1.5 50 - IO0

=, 1.5 I00 - 200

greater cost. ~ munit ions are often stored for lengthy period=, under conditions of normal atmospheric cycling of tempera ture and humidi ty , haterest at Ma~fial~ R e , arch J'.~boratories (MRL) has been cent,red on the pro~,ective properties of the~,e films. To understand the corrosion mechanisms, basic knowledge was required of the formation a',¢d aging properties of ~he films.

This paper outlines the d,~¢eloprnertt of chromate conversb n coating,,~ on zinc and dmcusses their mecaanism of format.ion and bash: st ~cture. The mode of corrosion of chroma:;ed zinc electroplate is presented as we~l a:; a discussion of accelerated corrosion test m,:thods.

Deve~.opment of the chromating process

After the early discovery by Wilhelm [ 1] , Anderson [2 ] made a com- prehensive study of the format ion of chromate co~wersicn coatings on zinc. He d e v e l o p ~ the well-known Crcnak solution [200 g Na2Cr..~OT-2H20, 6 - 9 mi H2SO4 (s.g. 1 .84)and 1 I of H20)~ which operated at ~oom tempera- ture with immersion times of 5 ! 5 s These " - . cnr,., mate eonwrs ion coatings were an iridescent-yellow cotour. Other ch_romatmg solutions were developed later but these continued to rely on an ac,di~' ~ e dichromate solutio~ with added at.iotas [3 ] such as sulphate, c~loiide, fo!,mate, phosp?ate ~n,d fluor- ide. As ',hown below, these anions h,~e a s~ectacu!ar effe~;t o.~ the rate of formatAo~ of films on~ zinc [4 ] . The advaniager~ o f v~ ious prepri,etary seiu- tions are ~ot cle~xly r e ~ r t e d but , in those results publish,.~i [ 5, 6] e~d in unoublished work by Esmore ~ d Lindemayer , t he C~onak pruce~s was equi- valent in "~-~ ' c .mos~on preventio,~. ~o 1;he proprietary ~ l u t i o n s testc~O.. ~ m e of these res,,lts are s how~ inF g.. ! in whwn the t ime ~ failur~ in a salt spray test of r , ropr ie t~y and Cronak proce~e~ i~ co~pm°ed. Adva~ces in

T A B I , E 2

P e r c e n t a g e c : , m p ~sitio~i <)f a n u m b e r o i ( 'h r ( )n , :~ : ( ' c,:~)~,,~ , ,~,.>~

c o a t i n ~ / s ,n z i n c

C ' , r o o . a k ,..r~,Y3, ;~a.*. '.

C h r o m i a m ( V [ ' ~';.7 T ! '2 3 "

C h r o m l ~ u m ( l l )" 28 2: , - 3t) 2~', :2 ~. ,,~ . #

~u p h a,, ("

C h t¢)r ide

Li lac

W a l e r

R e % r e n t ( "

3 . 3 21 - 3 , ! ,

'2. I '2 .~ 1 2 . 5

1 9 1 5 - ~ . )

l'- ~ 1 l ~'71

' I

Ii

,!

• !

": i

: ' : q i l h ~ & ! , , : z ( % :

!

A!t,i.).o~.~gh t h i s a l k a } d ~ , , .......... , . . . . . . . . .

108

d

~2

O* 02 03

Concentration, Na:~RQ~, mol/l

Fig. , . The effect of sodium sulphate addi t ions on the weiglht of • film formed and v zinc d issolved ot~ a 120 a immersion in 0.12 m~l I I , o~ Cr ~a solut tons at 20 'C,

Tire m e c h ~ m of formaation of chromate conversion coa$ings

This is one area which h ~ received very l i~le attention. Since the early work of A n d e ~ n [~] ahd .Clarke m~d Andrew [7] there have been few publications on the rate of formation of films [8] or on the effect of added anions on the chmmating reaction [ 4, 9] . Chemical analyses of the fihas formed on zinc (Table 2) are consistent with the following reactions:

Cr~O~- + 14H" + 6e -, 2Cr a" + 7H,,O (1)

Zn-* Zn ~' + 2e (2)

Tl'e reduced Cr a" is precipitated on the surfa..e of the zinc probably as a hydrated oxide. Zinc is only found in ~:ne film in trace quantit ies and normally accumulates in the chromating solution. These reactions, however, are dependent on anions which do not appear in the above ~quations. For e×ample, in pure CrOa solutions it is well known that zinc does not dissolw:, to ~r,y a[ preclable extent. On the addition of sulphate (Fig. 2), the zinc dissolves and a visibM coating is formed on the zinc surface.

All recent evidence f4] supports the proposal oy McLaren et al. | 10 ] , that chromate conversion coatings are hydrated cbromium(iYt) oxide films with adsorbed anions such as chromate and sulphate. Nevertheless, the early suggestion of Anderson [2] , tha t the film consists of a basic chromium chromate, has appeared in some recent publications [11 - 13] . The chromium(Il l ) hydroxide should precipitate if the pH in the vicinity of the zinc surface is approximately 2. Although ~he pH of most chromating solu- tions is lower than this vatu~ tha hyd~oger~ i~,,~ i~ ,.~.o..,~...~ ~ .......... ,:~-- and the pH change at the surface permits the preeipi~Ltion of CrIOA)a.

This surface pH value is reported to have been measured for a 0.07 mol 1 ~ (~(VI! solution containing a d d ~ sulphutic aci'd [14]. At low H2804 concentrations, a p H o f ! , 7 5 ~ me~ured ~ m ~ i i a t e t y on immersion and this was n,~in~ined for ~ m e min~t~,,i~At-hi~er.H~SO:~ , . . . . . . . . tone-at ;rat ions the pH decre~ed linearly v~ue . For example~ ~dth 0.02 mo t o stabilize at the zinc surface.. This :~otk a l so ~iaowed tha t t h e pH was : inde~ndent o f

T A B L E 3

E f f e c t o f h e a l i n g ()n chromat-,~ ' e,)ating, s [ ~ r, ]

T e m p e r a ~ urn' A p p e : ' r a n e ¢ ' af!~'r 9f~ h , , e a c h a l ) l e ( ' r ( V |~ ~t" d r y i n g ,salt s p r a y * ~ ~ fi ~m

,,pg c m ,

Z n Cd Z n

R o o m NC NC 20 50 TC NC 2() 75 SC S(: 13

1 O0 C ,; 0 .3 150 C C

* N C , n o c o r r o s i o n ' " , ; , ; FC, t r a c e c o r r o s i o n b'(? s l i g h t c (~ r ros ion C general c o r r o s i o n .

whether cast or plated zinc was used. It is unfortunate that Aub()4 addition. were used during these experiments beeause, as already indicated ~ Fig. 2, t h e s u l p h a t e ion as wel l as; i.he h y d r o g e n ion d i r e c t l y a f f e c t s the ch:'c ma~in~ reaction.

T h e role o f a d d e d an ions dur in l i chromat ing , ~s b~triguin~. Tl~ey at(, u n l i k e l y to affect, d i r v c t l y e i the r o f th..e red~>x r~,a,:tions a~d pr~bat, , iy :~,c~ t~v e i t h e r c o m p l e x i n g w i t h c h r o m i u m ( t l ! ) in the, fi lm or a l t e r i ~ th:, i;~"-~:i:" r~ i<,-. o f t h e p r e c i p i t a t e d c h r o m i u m ~ l l l ) t~vdroxul~ by a<l~<~r~ ~ , ~-.:i.-.,

Fig. 2 d o e s no t (~ccur wi th th(:. c h l o r i d e ion 14] . :ks t h e stai:,iiit) ('(~:~sla~,:< for ch romium(I :~ l ) c(:)ml~i(:,x. ~,"~ o f (:1 and S(,~i" at' ~, si~nitar ~h,t,~~ i"~ '°, ;~, , ' o m p l e x i n g r e a ( t i o n was the mai.u reas~:;n f()r the ~ha;~(, ,:)f ~.h,, , . ~ r v , , , . :~

m a x i m u m wou ld also he e x p e c t e d for Ci add i t i ons . 'i'hie~ skaggosls ~. ha;i. t i~( ~ s u l p h a t e o r . h l o n d e is i n ( o r p o r a t e d in t h e film bv a d s o r p t i o n si,~,;lar ~) vh(, m o d e o f i n c o r p o r a t i o n o f c h r o m i u m ( V i i . The welt~ka(1)w~ dif[eren(:( ' i~:

qr~° a n d C1 ", ~ acc ,~unt f:)r ~' a d s o r p t i o n p r o p e r t i e s be',.we~,n ,,,.,.~ tm . . . . . . . . . . . ::~-:, d~f f~~~.0 . in f i lm we igh t v e r s u s con~:en : r a t i on o f a d d e d an ion cur-ecs ',-~ i. ~Fl~is e:, !~tar.a t ion is f u r t h e r r e i n f o r c e d by t he c h e m i c a i analys,.~s in Tagi(, 2 wr~('h s h o w t h a t a h igher p r o p o r t i o n ()f SO~ t h a n Ci is retain(~d in ~:.hc f:.;. ~ . ~ .

w h e n n e w l y f o r m e d . Thi~ can be (~verc,.;me by air d r y i n g for 24 i~ or ~y f o r c e d d ry ing . I h e t e m p e r a t u r e at whic! ~, t h e ' r ...... ~ ' :fi~.a~-~;iv affects the pro~ective t~roperties of these coatings degwnds on ~h,:, t~a~ ~ ,,! h e a t i n g a n d t h e m e t h o d used to evaluat ,e t he c o ~ ' o s i o a rvsis~ance. ~~,r

s h o w n in Table 3 and t h i s b, as be~:,~, r"--~'~.m'~"~ L, ~ .............. ' .....

110

panels were subjected to excessive hea~, the chromate was not leached from the film during salt spray testiag and there was an accompanying loss in cgrrosion resistance. However, when the samt pmaels were subjected to an outdoor.2xposure test, the corrosion resistance of panels heated to even 80 °C for 2 h gave similar p~rfotmances to air dried panels [ 16] . The weli= established procedure [ 3] of restricting the drying temperature has some h, undation but , in many instances, and particularly with type A m~d B films, higher temperatures may be used without any detrimental effect on the protective value of the film.

Heatint4 to 150 °C results in the formation of ~ 2 0 3 , as determined by X-ray analysis.., and, if the film is still in contact with the zinc metal, zinc chromate is also detected [2] . This effect was noted for free Cr(OH)3 as early as 1901 [ 17 ]. If heating of the article after chromating is unavoidable, e.g. for relief of hydrogen embr i t t l eme~t post- treatments with sodium sili- cate, barium nitrate or glycerine are., reported to be partially successful in preserving the corrosion resi~t~n,ce i[ 18]. However, it is usual to form the chromate coating after her. ~ing, when it is customary to reactivate the zinc surface with a short dip i~:: an acidic solution [ 3 ].

Once the films are formed, they ¢'.an be cracked by heating, storing in dehumidified air at room temperature, placing them m a vacuum or placi~:g them out in dry' sunny conditior,s for ,>ne day [ 16]. The cracking of the film is shown in Fig. 3 where an air dried film (no cracks evident) was ulaced ~n the scanning eiectton microscope. D,.spite the appearance, there is n~, degradation in corrosion protect ion associated with simple cracking of ~he films either in the salt spryly or cu tdoor exposure test [ i 6] .

Corrosion protect ion of chromate conversion coatir,:gs

As already mentioned abo~e, the comparative corrosion protect ioa of these films depends to a large extent on the type of environr~ent in which they are exposed. For example, heating of the coating affects the response to a salt spray test but not to a~l outdoor exposure test. An understanding of these apparent anomalies re<luires a reasonable knowledge of the mechanism of the protect ion of the zinc by chromate coatings.

In protecting the underlying zinc all chromate conversion coatings act

C coatings, is typically 0.1 - 0.6 pm thick. For coatings which contain only minor amounts of Cr(Vi), i.e. type A and B in Table 1, this i,~ the main mode of protection of the zinc. A typical time to failure in the salt: spray test for such coatings is 24 - 48 h.

Type C and D coatings also contain chromate which is adsorbed on the Cr{OH):~. in aqueous medium this can be leached from the coating am.l can lead to a repassivaLion of damaged areas on the surface. This ~ate of desorp- t~oa of chromate aopears to be dependen~ on the history o: ~ tlhe coating. In an accelerated test like ~he ~ t t spray., the deso~ption occurs readily wi~h air

11

(a) tb)

(c) (d} Fig. 3, Scanning e!ectron mierographs of c!~romated zinc pane|s: (a) air dried ::~1 ~~::~~ (850 x ) , ( b ) a n d (e) dal :ksp~, tson the surface after ~.7 b ~a~t spra?' ie:;tit:~: ~:r~d ~d}l.i~ :,,.~.~ spot as (b) with the eorrosio products relnoved {,: 150).

d r i e d coa t ings d u r i n ~ t he f o r e d a y s o f the t.est~ 1~~ ~'ompa~isox~, ~he ,..:~(.~v~,~,~ d o e s n o t d e s o r b froa~ coa t in~s w h i c h have been ~;.~te~i to SO (~ f~~ 2 }~ i !~ i} In o u t d o o r e x p o s u r e w h e r e t h e l e a c h i n g is m. t~ch s low ~'~, .:. ,~ ~i:~ ~ . . . . . . . . . . . . . . ,~l.~(~r:c~ ~:: •, ;,~"~. .... ~ ~f

w e r e hea t ed [ ~ao°'j o It is easy to u n d e r s t a n d t h e p r o b i e m s <,f accei~:rat~d <:~::,r~ ~s~.:~-,. i c - ~ ~

c f ? y p e G and D c o a t i n g s {air d r i e d ) as r~p~d i e a c h i e g o f t, he cb:ron:~a~.e ir~)~~ t h e f i lm i~ r e q u i r e d , This is n o t a n o r m a l circ~':~si:ar~ce, as corrosi':~v~ ~:i ~ t}-~ ~ c h r o m a t e ~ zinc e l e c t r o d e p o s i t m a y t a k e ~o:~t~;.s o r eve;~ year.:.~. 3:h~s. ~:orr~,[a- t i o n s b e t w e e n a c c e l e r a t e d tes t m e t h o d s anP, :0~,~ua! o~.~tdoor ~",v~:,s.~..~r,~:: a '~ mi s l ead ing ] un,~.ess" t he r e is a c~o~. : ' a,~ ' e ' ~ , . . . . . . . . . . . . . . ~.-v~,~,~,~

in t h e a c c e l e r a t e d ; e s t and 'l:he a c ~ . a ; : : : ; - :sur~ .;i~,e~ 1-:k?r i~a tance , Kat~z e t a t .

f 2 0 ] r e ~ r g ~ a g o o d c o r r e l a t i o n b e t w e e n pane i s e x N ) s ~ in t h e salt. sp ray

te,,~t a n d d u p l i c a t e s e x p o s e d c lose t~:~ b e a ~ : : : :

112

!Ill .... II ¢ O | ¢

Fig. 4. The ~ver~qI~ t : i~ to mpl~'~van~ of white ~me~. ,a produc~ it, the ~alt ~pray u~stmg of six different batehes (A - F} of ~mmercial zinc ~tectroplate, ~howia[ the var~tio~ in :re~ult~ f~om four dif fe~nt ¢ o ~ ¢ ~ a l a ir ~pr~y cabin~t~ [ 221,

The first s i ~ o f corros ion o f chromated zinc ~n the mlt spray te~t [ 21 ], '~he humidity test a:~,d outdoor exposure [ 161 is tt~e apl~2arance ot black spots. Th~ is due to corrosion under the chromate conversion film~ ~ shown in Fig. 3 fear spots produced in the salt spray test. 'l'he chromate coating usually covers the corrosion product, as shown in Figs. 3(b} and (c}. The corrosion product can be seen in Fig. :;(c ). 'When tI',.~ corr¢)sion products are removed, F~g. 3(d) shows that corrosion of the 13 ut]~ l~hick 7h~c electro, plate has occtu~red through to the steel surfa~.'*:.

Salt spray t~ t s

Most :;alt spray tests conform generally with th<- ASTM B117 standard test method, except ~hat the volumes of the cabinets are often less than t~e 0.43 m 3 (15 ft "~) specified, t6~ho:~gh the grading of panels is comparable from one cabinet to apother, the absolute time to failure cannot be comp~ed accurately from one cabinet to ~ g t h e r . The resuits of two series ~f comparative trials ef salt spray cabmets have been published [ 22 ] and ~hc w variations in t ime to failu~ of up to i000%. The histogra~a (Fig. 4) :shows figures for electroplate prepar~i by ~x different ptate~ and tested by focr different laboratories, Similar in~x~nsislencies have been re~<)rted wher~ cor~parat;v,~ s~lt spray test trials have been conducted [23] .

The ~easons for the lack of correL~.~on have not ~ e n resolved. 1~ was ~eaerally f~tt [ 16 ~3] t~h~t the inco~sistencies were caused by impurity :te~'els in the s~t, the w a ~ r or '~ ~ tae am ~n an effo~ to el~im~nate ~wo of these va::iabla:, a third comme~'cial e x ~ c i ~ ~ c o n d u c t ~ by Materials R ~ a r ~ h

2

O

~, tO0 . . . . .

o v w .~: v I

Fig, 5. The aw.rage t i m e to , q~pea rance o f wh~tc curc¢~sion / p ~ d a.:ts o n ~;etb ,~I-s~ pa~+'~.~ tZinc p l a t e d a r d c h r o m a t e d ,n ~,ne bat .oh) t'¢)~ d u l ) l i , ' ~ e 0',~I,e~" men~.~ ~n ~'ivc ,~if;er~'n~ ~.;~!'

5

2

s~andard steel p; ,~ets(,n a 2 h ~. imuttant o, t~s+':~:g)~s~arc i: t h~cab in~ . I s

p l a t e d a n d c h ~ o m a ,ed in o n e . . . . . . . . .

was d isso!vc<! in die , t i l led ws,.,,,r to m a ~ e t,:," 5 , ; ~ Jwi , ~ .~,~,i,,~.~. ~

e f f o r t w a s m a d - ,,:) s t a n f l a r c l i z e ,.he o ~ + r a t o r s ir +hei r .... i + -, ++ ~ ....

s t e e l p a n e l s w e r e a l so ex l :~)sed in t h e s aig s p r a y ~:abi~-~t,~,; ~ w~ i~:?, ~,~;-~

~v+.,re d e t e r m i n e d f o r a 2 4 h p e r i o d .

T h e resu l t s o f t h e tr ia l ar~ c o l l a t e d ir~ ~'i~,~+,, 5 ~ , ~ .... ~ ~'~ ",~~,~, ',: :~I ,~ : /

a c h i e v e d p r e v i e u s l y ~see F ig . 4) . S ix z i n c ~.,~o~: ~ w~ ~'c c.,,p~,~cd ,~, , , ,~; , c a b m e ~

114

and the results axe given ~s a r~. average time to failure. Duplic~Le tests were conducted in each cabihet. The differences in t ime to faiture between the ~two runs in each cabinet ranged between 0 and 40% whilst i.he ave~'~:~e time for ail experiments was 181 h ~ i th a sta~dard deviation of 48. The worst :individual results were 47% high and 39% '~w when compared with the average. If values o f 96 or 72 h are to be used rigidly as standard values in the salt test, then even the values obtained under qui te well-controlled co.~di- t ions {more stringent than the A~,TM Bl17 S tandard)a re not sufficiently reproducible.

The comparison between the t ime to failure for the zinc plate and the weLght loss of plain steel panels can be ~een in Fig, 6. When analy~d by linear regressio~ a value of the coefficient of determinat ion (r ~ ) was 0.12, confirming that there is no s!gmf cant correlation between the two variables m Fig. 6,

The ASTM Standard Bl17 repox~s that periodic fluctuations in air pressure of 3.4 kPa (0.5 psi) can result in a two-fold increase in corrosivity ,of the fog as shown by the work o f Damey and Cavanagh [ 25 ]. On close ,examination of their paper, the corrosivity (as measured by weight loss of

m . w t one ~eel pane l )on ly vaned stgmflcan,ly in one of the four experiments where a 1,63 mm air nozzle was positioned 0.13 mm below the tip of the fluid nozzle. This one point is no t repeated in the paper even when bet ter ,control of the pressure was achieved i- a second series of experiments. The problem of air pressure changes or so called u ritical pressure effects has not been detected in any of the trials cor~iucted by MRL. Other dot~bts about l:Le reproducibility of the work of D~:sey and Cavanagh [25] are raised by l:he fog collection rates. For example under supposedly identical c~nditions i~ the one cabinet with the one nozz;e, the fog collection rates for f~ve ¢lifferent runs varied as follow~, • 0.9, ~;1, 3.0, 1.0 and 0.8 ml h 1 The critical pressure effect has no apparent the~.retical explanatic, n and some confirmatory work should be a t tempted. In the third ~riat conducted by , ~ [24] the weight floss for the s t~4 panels varied from 0.14 - 0.19 g/day for the nine different experiments. Therefore, the lack of re~)roducibflity i~ the results for the zinc plated panels cannot be ~)xplained by a .nt~cal pressure effect.

In summary, the salt spray test - ' ~" accozdmg to ASTM Bl17 is not a repro- ducible test for the evaluation of chrcmated zinc el .ctrc p,ate on steel, even wh(:n the quality of the reagents is st~=tctly controlled. Ti~e variatior~ may be ~utr t,o pu~uuanu~ it~ ~,nu a~mospn('re ~.:r may restll~ trom :~uor~le cll~lerences in the storage conditions under which the panels are distributed to the ~ arious laboratories and stored p~ior to the test (i.e. the effect of aging of the films). In any event, the actual times to failure are misleading when trying to assess a partic~dar item for ou tdoor exposure.

H u m i d i t y t es t ing

This tes t c lose ly a p p r c . ( i m a t e s t h e t y p e of corrosioi.~ (~x.t:(~ri,~,,:ed :~ a l r op i cM e n v i r o ~ m e ~ ! . It a lso. t,r~,~bably m o r e r,:~alis~i('a!ly, roI,!°~,s~ , - ~,~ ~ .. o f corrosi(~n in p a c k a g e s which aro st.r:,red t:,~(~,r ~,'~,~,ii'~i~,~- ~,2 . . , : ; , t e m p e r a t u r e and h u m i d i t y . Var ious h u m i d i t y - t e m p e r a t u r e cyctes h~ve b e e n used i~ t he genera l t e s t i ng o f zinc, a l t h o u g h the A u s t r a l i a n S~an ~rc~ 17 c 1- 1 9 7 6 d e f i n e s or . ly o n e sot o f c o n d i t i o n s (16 }: at 55 °C a n d a m i n i m u m o f

o/,-, \ 5 h at 35 ,~,. ')t'he D ra f t I n t e r n a t i o n a l S t a n d a r d I S O / D I S 3 6 1 3 res t r i c t s ~he t i m e o f storage' b e f o r e t e s t i ng of c h r o m a t e d zinc to w i t h i n t h e range 24 . 7?. h lh -esumably , th is is t o a l !ow t ime for t h e coa t i ng to d r y ou t u i t h o u t a l lowi~g a n y s u b s e q u e n t chan~e~ m p r o p e r t i e s such as l each ing ra te . H o w e v e r , these c h a n g e s in t h e f lrn d e p e n d on t e m p e r a t u r e and h u m i d i t y as well as t im e . In a ser ies o f tes ts ~vhere t h e panels t o o k severa l mon~:hs to pre- pare a n d ~ e r e t h e n s t o r e d for 3 - 4 m o n t h s at t e n , tarot t en p e r a t u r e , n o n e of t h e pane ! s e x h i b i t e d a w h i t e c o n ' o s i o n p r o d u c t af t ?r ~1 dag~ ~,~?o,a~r~ l(~ ~,,, h u m i d i t y tes t ['~61. T h e c h r o m a t e was n o t leachec, f r o m t.i.~: i~,m a~ .,sho~n by differential .~ulse p o l a r o g r a p h i c ana ly s i s o f t he . :onde~zsed wa t e r Col lec ted as r ,m o f f f rom t h e pane l s . These r e su l t s c o n f i r m t aat aging, of the f i lm ca~ have a d ra s t i c e f f ec t o n t h e s u b s e q u e n t c o r r o s i o n r e s i s t a n c e of c h r o m a t e d zinc e l e c t r o p l a t e .

p la te in t h e humid i t ? ' tes t bu t , as pane l s have to be s~:~.~.~:~,,~i t(~ ~i~, ~,,st withi~a '~,2ch strin~er-t t i m e l imits, th is t es~ m ~ , t l , . ( ~ l ~. ~" ~. . ~ ' . . . . . . . : =

( ; h r o m a t e ah~ ty ~is

in an ear l ier p u b l i c a t i o n [ 3 ], it was s ta ted tt!~t "'th(, ~'hr~ ~-,~a~.~ a~.~.:.~i\~, seelos to be m o s t p r o m i s i n g (e~ a q u a l i t y c o n t r o l t~s' t bu t a t ,.~:~i~,vai,i ,~ a m o u n t o f inv.estigator / w o r k m u s t be ux~dertak~'; !~,~.,f(~r¢° a ~:-~ti,f~,'~,,r'., s t a n d a r d can be establish,~d ' ' . A f t e : a n u m b e r of e~ .per ime~ts , ',r a ~ l ~ , , ' a ~ "- ~, ~ o , :

t h e . c h r o m a t e c o n t e n t o t h e f i l m a l o p e does ~o t det(" r .min~ ° tl~.(' ! ,',-)t~.,- : ,

value o f t h e film, Ear ly resul ts by Katz e t ,'.~] [20,] a ~ !;~t~ ~r ,~ ,,.., i :~ ' ' s h o w e d t h a t , for z inc pla"~e prepm"ed at the ~.)~.~' ~i~,~,. ~i~.,~.r,, ~ , :,~ . . . . ~: b e t w e e n titan, to fa i lure in the salt s p r a y tes~ a~d ('i~r, .rr, a~ .," " :;~ ~,~ ,~,:' z inc s a m p l e s were p l a t e d by a ~.t, mbec ()( cliff, ~ r ~ :~');,~,~ ~'~ ;~

s p r ay t, es t (see Fig. ] ) a n d the re.su!Ls mdi , ' a t , , ~.hat Li~., ~.~',~-~ ~;.-,- ,. . . . . . . . .

d e p o s i t m a y well be a rr a jor c o n t . ~ ' i b u t ~ f a ' ~ ) r . M ( ~ r ( , : r , . : e r , , , v t ~ , ; ~ : ~ . : ; " , , ~

were e x p o s e d o u t d o o r s , it was ~o t i',()ssibi( t~~ m a k e :~.~v (,~rr~.~,!:v~- b e t w e e n c h r o m a t e con te t~ t and co~os io~ , r~si ~ta~'e. cwei~ ~ t i~ i~a~,,~[~ i ' i~, ' ,~ u n d e r i.~ent~,ica~ c o n d i t i o ~ [ 1 6 ] . IL is int.er('.~li~u to :~)t(, i~. Fi.. i ti~a~, a l t no t i gh sc, m~, pane l s f a i e d t,h~ ci~ro~.~at~, a ~ . . ~ ,

t h e y d id n o t ~ecessa r i ly fai! ti;e sa![ s~.~ra>

: t t 6 ' • •

~etefom, although the chromate ¢~ntent enhances the protective .value of chromate ~nversion films, it is not a good me&sure of the corrosion

* e resistanC, The~~ for thin ~e that aging affects the ext~ction of the chromate from the fl mand the profiles of the zinc plate appear to be a :~ajor contributing factor in determining the corrosion of chromated zinc

Conclusions

Iridescent chromate conversion coatings are still si:m~Ia~ ~o those, ori~s~y ~u~: by:bmd~n,[2] and the major advance in moderns. pro:prie~ ~ ~ i o m m-~h~ the time o f immersion : ~ . ~ al~red to suit;

i.e,Lty~ A c~tinga)are a more recent

~Ihe ~ ~ : 0 f ~ e , : , © ~ ~ t m g . p r o ~ invoh~es the precipitm ion of Cr(OH)~ with t ~ . ~ l s ~ t ~ n of anions such as chromate in the fihz,. More ~ o w l ~ g e o n the mechanism of the reaction is need~, particular y wit! m~otion.

is on both tt e ~amer !ay~ of the chromate ,,~sorbed in the .~iim, ~ t~ingofthe film affects the d~~tion of the c.hr<,Im ~, ate and makes ~ difficult to predict the protective value of these films. Accelerated corrosion tests such as the salt spray ~st do not produce reli;~ble ,results and do not correlate with outdoor exposure tests. In general, chromate content of the film does not correlate with corrosion resistanc~ of the chromated zinc electrcplated by a number of firm~. This leaves the corrosion testing of chroraated zinc in a very unsatisfactorf state.

References

3 4 5 6 7

9 10

12 13 [4

t5

T.

E. J. Wilhelm, U.S. Paint, 2,035,380 (i936). E. A. An der~)n, Proem Arnec Electroplaters' Sot., 31 (1943)6. I,. F. (3. Wil{iams, L. F. G. Wi|~iams,~ ~76) 355.

Biestek, Ft. tnSt, :Mech. Precy~, ',15 ( t967) 47. E. Halts, M e t ~ ~l~eat., 14 ( t 9 4 7 ) 1 6 4 ,

G. Woldt, Werks~.ffe Korros~, 1-2/{:!961 )486. ~, I. L+~vitin~, ~ . Fi: .: :.ram. 34 (~t960) 2075. K. G, McLa~en, ,J!,i H.,G~ ~ -~ SoL, I (1961) 161, I%). M. E. Rope~. Me. i~I F, mshm~ J , , H ~ ~ . . . . . .

Ele~ A.[

~'in;~hin~, 4.2 ( t 964) 163.

117

16 L. F. G. Williams, to be published 17 M.J . Feree, Bull. Soc, Chim., 25 (1901) ~)20. 18 F. Pearlstein ~,nd M. R. D'Ambr~sio, Piatin~, ,,7, (1968)3~5. 19 L. F. G. Williams and B. q.'. Moore, Mat(--ials Research L~boraiorie~, ile~). N,~. 7, ] ..

(19"72). 20 it L. Katz, K. I,. Proctor and F. Nagh°~,, Proc J,m~, ~(~c 1~ .~ , : , ~:~¢.~i~ ~;

(1957 ) 203. 21 L. F. G. Williams, Corrosior, Se;.. 13 '1973) ;J~5. 22 L, H, Esmore and B. D bindenmay(.r, Etectrorflatin~ Metal Finishing, 26 (Nov)

(1973) 18. 23 E. H. Cornish, Standard Telec~t)mmuaieation Labor;i~.ories~ L~)ndot~, p~ivate

communication. 24 L. bI. l~more, Materials Reseaxch La,)oralories, iX'lelbo~lrn¢,, p~ivate cornrnunica~.~on. 25 V. M. Darsey and W. R. Cavamgh, Proc. Arner. S(,c. T~,sting Material:~, 48 (1948) 15:; 26 L. F. G. Williams, paper presented to 6ti~ tnt. Congr. on Metallic Corrosi(~n, Sydney.

1975. 27 F Taylor, Metal Ind. (London) , 65 (1944) 1 49.