Investigation On Microstructure And Electrochemical Performance Of Al-Zn-Sn Series Anode Materials

Al-Zn-I n series alloys ar e widely used as sacri fici al anodes for cat hodeprote ction of ste el compone nts in t he seawater due t o their high currente ffic i e nc y a nd u ni f or m s ur fa c e di ssol ution morphology. However, with theproduction and using of indi um, peopl e graduall y r ealize d that i t i s har mfulto t he environment. At present, ma ny researchers are worki ng on the Alsacri fici al anode wit h hi gh-perfor mance and pollution-fr ee, and a number ofnon-indium anode mat erials have been de velope d.The curr ent effi ciency of Al-Zn-Sn all oy is l o w, the c orrosion productsadhe re on the alloy sur fac e a n d the c orr osion mor phol ogy i s " metal s ponge"-like. In order to i mprove t he el ectr oche mical per for mance of Al-Zn-Snano de all oy, t he i n flue nce o f G a, Bi, M g a n d Ce e l e me n t s o n t h emicr ostr ucture and electrochemi cal performance of Al-7Zn-0.1Sn anode wasinvestigat ed by means of mi crostr uc t u re obser v ation and electr ochemicalmeasur ements in t hi s paper. The results show t hat trace Ga can signi fi cantl yi mprove t he electr o chemical properties of Al-7Zn-0.1Sn anode. For example," met al sponge"-like mor phology in anode sur face disappeared, corrosionproduct s fall off e asy, the phe nome non of hydrogen a nd grain off wer ei mproved, the cur rent effi ciency increased from76%t o95%. A ndAl-7Zn-0.1Sn-0.015Ga alloy with a curr ent effi ciency of96.4%wasdeve loped. The n, t h e furt her opti miz ati on wit h Bi, M g a nd RE wa s c arriedout on it, Al-7Zn-0.1Sn-0.015Ga-0.1Bi all oy wit h a current effici ency of97.4%and a mor e uni for m-dissol ution was de ve lope d a lso.To furt her i mpr ove t he diss oluti o n morphology a nd over allper for mance of Al-Zn-Sn s er ies anode all oys, t he sol ution tr eat ment andanne aling treat me nt were a pplie d t o the all o ys Al-7Zn-0.1Sn-0.015Ga,Al-7Zn-0.1Sn-0.015Ga-0.1Bi and Al-7Zn-0.1Sn-0.015Ga-2M g, i n flu e n c e o fheat treat ment on t he electr o chemical per for mance of Al-Z n-S n ser i e s a no d emat e r i a l s was s t u di e d. T he r e s ul t s s ho w that annealling treament couldma ke t he hydrogen evol ution rate and the c orros ion c urre nt de nsit y of theanode all oys i ncre ased, c urr ent e ffici ency de cr ease d, a n d the mor phol ogywas non-uni for m diss oluti o n. Soluti on t reat ment c oul d make the anodi c pote ntial nega tive shi ft, hydroge n e v oluti on r ate and c orros ion curre ntdensity of the anode all oy s red uce d sig ni fi ca nt l y, alt h oug h t he curr ente fficie nc y incre ase d or sli g htly decr ease d, but more uni for m c orros iona ppe a r a nc e was o bt a i ne d, so the anode alloys s h ow a bett er c o mpr e hensiveper for mance. Afte r soluti on t reat ment for three alloys, the work potentialwer e-1.05V (S CE) or s o, t h e cur r ent e ffic i e nc y wa s hig h e r t ha n93%, a n dthe diss ol ution of a nodes wer e uni for m.Effects of grain size on t he electrochemical pr operties of t he Al-Z n-Snseries all oys were inve stigated by means of cold defor mation and heatt r e a t men t met h o d. The r e s ul t s s ho w t hat t he re l at i onship of grain size andcurrent effici ency was not s i m p l y t h a t t h e alloy with s maller grai n size ha sthe hi gher current efficiency. I n the case of the same heat tr eat menttemperat ure (t he gr ain size was changed by “d i ffere nt defor mati o n+sa meheat treat ment”), t he curr ent efficiency of anode alloy first i ncreased andthen decr eased wit h the decrease of grain si z, namely, the all oy wit hmoder ate grains ha d the highest cur ren t effici ency. I n the case of t he samede for ma ti on (t he grain size was alte r ed by “80%de for mati on+di ffer entheat tr eat ment”), for the speci mens wit h non-uni for m struct ure, thespeci me ns wit h s maller grai ns had the hi gher current effici ency; for t hesamples with homogenous structure, the samples with s mall e r grai ns had thelower cur rent effi ci ency. Grain size had no si gni fic ant e ff ect on the anodesur fac e di ssol ution.The s ha p e a nd si z e o f sec o nd p ha s e of anode alloy were changed bysoluti on t reat ment and di ffe rent a ging treat me nt methods. I n fl uences ofsecond phases on the el ectro c he mi c a l p r oper t i e s o f Al scar ifical alloys werestudi ed. The resul ts show t hat t he shape, number, siz e and dist ri buti on ofsecond phases i n a node all o y ha d t he great i mpact on t he per for mance ofaluminum sacri fici al anode all oy. The anodes wit h rod-like and chai nsecond phases were easily corroded along grain boundaries, resulting insecond phases a nd undi ss olved grai n s hedding, r educ eing the anode c urrente fficie nc y and maki ng t he uneven cor r osion mor phol ogy. I n compari son, theanodes with spheri cal, disca l or bl oc k sec ond pha ses had high curre ntefficiency and even corrosi on morphol o g y. I f t h e s e c o n d phases were toosmall, t h e acti vati on r ate of anod e alloy was sl ow, easily led to thepolari zati on of anode all oys. The larger second phases were conducive t o activation alloy and impro v e ment t he e l e c t r oc he mical behavior of anodealloys. While t he oversize s e cond pha s es was not conduci ve t o i mp rove theelectrochemi cal behavi or of anode alloys beca use the s ec ond phase s e asil yshedded off and led to the decrease of cur rent efficiency. The all o y wit habunda nt second phases ha d l ower c urr ent effici ency due to the gr ain andthe undissolved second phases falled off. The second phases at t h e i nnerg r a i n wer e c o n d u c i ve t o t he a c t i va t i on a nd e ve n corrosion of alloy. Wh ereast he sec on d ph a se s on t h e grain boundary could lea d t o t h e l o n g i t u d i n a ldeve lop ment of c or rosion along the grain boundary. The anode all oy withthe me di um number and size, inerr atic shape a n d uni for m dist ribut ion ofsecond phase had the high current effici ency and t h e u n i fo r m c o r r o s i o nmo r p hol o gy. S uc h as Al-7Zn-0.1S n-0.015Ga-2Mg alloy with uni for mdistri buti on of400nm, di sk-sha ped s e cond phase had t he stabili zed wor kingpotential of-1.07V (SCE), i ts curr ent efficiency was more than95%, and it ssur fac e was very uni for m.Bas e d o n t h e s e s t u d i e s, the dissolution mechanism of Al-Zn-Sn seriesalloy was investigated by the cor rosion morphol ogy, el ectrochemicali mpedanc e spe ctroscopy testing in3.5%NaCl sol ution at di ffer enti mmer sio n ti me. T he results show that the initial dis sol ution of t h e alloy iscaused by pitting corrosion, and pitting started at the interface of secondpha ses a nd gr ain bounda ri es. At this time, a capa cit ive loop at hi ghfre qu e nc y i n N yq ui st dia gr a m c ha r a c t e r ized t he double-l ayer capaci tance ofalloy, the inductance loop at low fr eque nc y also c orr espond with thecharacterization of the relevant literature to expl ain the passivation ofmetal at pitting stage. Then, the alloy corrosion rapid grew lateralexpansion with two forms of pitting and dissolution-redeposition around thepitting caused by Zn2+, Sn4+, Ga3+. The i n du c t a nc e l o op o f t he EI S a tmedium frequency which characterize the pitting and the second capacitiveloop at l ow frequency which charact eriza t he diss olve d-rede posi t ion al soindicated that the corrosion controlled by the pittin g and dissolved-rede posit i on at this corr osion stage. With further increase of the corrosio ntime, the pittings were connected caused by lateral expansion, and fi nallythe entire alloy surface wa s uni for ml y di ssol v ed. Di ss olved-r ede positi ontook pl ace in t he entire sur face of t he alloy which is t h e major corrosionfor m at t he l at er d i ssol ut i o n sta ge. T he i nduct ance loop at low fr eque nc y disa ppe ar ed, the c apaciti ve loop with gra dual increa se at low fr eque nc ywhi ch characteri za the di ssolved-redep o s i t i o n a l s o indicated that thecorrosion contr olle d by the di ssol ved-r edepositi on.Corrosion potential Ec o r, pitting potential Ep i t, pitting change potentialEp t pand protection potential Er pof Al-7Zn-0.1Sn-0.015Ga all oy wereidenti fied by cycli c anodic polari zati on c ur ve s. C o mbi n e d wi t h c or r os i onmorphol ogy at feat ure poi nt s usi ng s c a nn i ng e l e c t r oc he mi c a l mic r osc op y,pitting initiation, expansion, and passivation of Al-Zn-Sn series alloy wereinve stigat ed, and pi tting propagati on m echanism of the all oy wa s re veale d.Location of second phase interface occured pitting preferentially caused b ythe combi ned effect s of t he Cl-a bs orpti on a nd t h e mi cro-c orros ion c ell. Thepits ra pi d de vel opment to the depth at fi r st, because metal cationconcentration within the pits increased, chloride i on c onc e ntrationincrea sed graduall y ca use d by t he mi gration of chloride ions to maintai nelectric al ne utralit y, and met al i o n hydrol yed due to t he met al i onconc entr a tion incr e ase within the pits. with the sec ond phase di ss oluti onand hole depth i ncrea ses, the ma s s transfe r resist an ce of metal ionsincreased caused by i n the corrosion product s accumulat e i n the corrosionhole, resulting in concentration polarization which led to the potential o fhole bottom shifted positive, t he corrosi on r ate in vertical slow down.liqui d Ga-Al a ma l ga m for med be caude o f t h e Ga i o b a c k t o a r o u n d t h erede posit i on Sn vic inity the pits, whi c h s epar at ed t he oxide fil m and thesubstrate, the α-matri x was exposed to di ssol ve c o nst a ntly, and thecorrosion products fe ll off unceasing also, the lateral activation extensionof pits were maintained.In addition, the relationship between the depth pitting and the acidityvalue within pit hole for the alloy was calcul ated for t he pi tting of Al-7Zn-0.1Sn-0.015Ga al loy oc curre d pa ssivat ion i n per pendicula r directi o n a fter itexpanded to a cer tain ext e nt, rat her than the traditional view with theformati on of deep corrosion hole s. The res ults show that the hydr oge n i onconc entr a tion i n t h e corr osi on hol e r apid incr eased wit h the increase ofcorrosion pits de pth, but t he change of concentration was ver y s ma l l w he nthe pits depth changed in the ma gnit u de of0-10-4c m. This de pth range i sthe "barriers" whether the pitting holes could continue to develop in dept hdirection. Pitting was passiva ted before forming the acidifi cation tank with high concentrati on because of the l arge deposit ion of re action pr oductsoutsi de t he c orros ion hol e, which failed t o brea k the depth " b arrier"ofcorrsion holes and therefore had not the ability to deep dig.