Nastanek in rast utrujenostne razpoke v korozijskem
mediju
Occurrence and Growth of Fatigue Cracks in Corrosion
Environment
L. Kosec*, F. Kosel**	UDK: 620.193.01
ASM/SLA: Rlh, Rle, R2j, Q26p
V	prispevku obravnavamo nastanek in ravzoj poškodbe kovinskih materialov v obliki klinov iz korozijskih produktov. Analizirali smo pogoje rasti in motenj v rasti klinov, vpliv števila in velikosti klinov na intenzivnost napetosti v sistemu kovina-klin. Rezultate analize smo posplošili s tremi primeri poškodb delov orodij in naprav.
1. UVOD
V	kemično aktivnih okoljih nastanejo na površini kovin korozijski produkti, ki so različno trdno povezani s kovinsko osnovo. Temperaturne ali mehanske napetosti lahko poškodujejo plasti korozijskih produktov. Tesno oprijete in goste plasti korozijskih produktov upočasnijo ali povsem zavro potek korozijskih procesov. Poškodovane plasti pa te zaščite ne nudijo ali pa zgolj v omejenem obsegu. Poškodbe so različne: razpoke, luš-čenje korozijskih produktov na posameznih delih površine ob meji s kovino ali znotraj korozijskih produktov.
V	prispevku obravnavamo vplive temperaturnih napetosti in oksidacije kovine na razvoj in obliko poškodb, ki pripeljejo do porušitve. Analiziran je primer, ko seje zaradi mehanske nestabilnosti porušila oksidna plast na površini in je skozi nastale razpoke prišel korozijski medij v stik s kovino. Na takih mestih je prišlo do pospešene oksidacije. Zaradi posebnega načina dostopa oksidanta so na teh mestih korozijski produkti zrasli v obliki klinov. Korozijski produkti se v fizikalnih in mehanskih lastnostih bistveno razlikujejo od kovine. Zato pride pri temperaturnih spremembah do napetostno deformacijskih stanj, ki vplivajo na morfologijo ter deformacijo korozijskega produkta in kovine.
Rezultate analize modela ilustriramo s tremi primeri: z elementom orodja za tlačno litje medi, cevmi pre-grevalnika pare iz termoelektrarne in anodnimi palicami akumulatorja, ki naj pokažejo relativno razširjenost pojava.
O pojavu oksidnega klina in njegovem vplivu na porušitev kovine je malo strokovnih referenc. Vpliv oksidnega klina na širjenje razpok je analiziral P. T. Heald1 in ugotovil, da razpoka samo zaradi oksidnega klina ne more preiti v nestabilno rast, če sistem ni obremenjen. Razprave o pomenu oksidnega klina na razvoj poškodb strojnih delov in naprav pa najdemo tudi v naši strokovni literaturi2-3-4-5-6.
The contribution treats the occurrence and grovvth of defects in metal materials in the form of corrosion pro-duct vvedges. The conditions of vvedge grovvth and crack grovvth retardation, and the effect of the number and size of vvedges on the stress intensity in the metal/ oxide system vvere analyzed. The results of the analysis vvere illustrated by three examples of this kind of defects on certain tool parts and equipment components.
1. INTRODUCTION
In chemically active environment metal surfaces get covered by corrosion product coatings vvhose adhesion to the base metal is variously firm. High thermal or mechanical loads of metal/oxide systems can impair corrosion product coatings. Firmly adhered and thick corrosion product coatings can slovv dovvn or even complete-ly stop the progress of corrosion, vvhereas the injured layers can no longer protect against corrosion or can do this only to a certain extent. The defects of the injured coatings can be of various kind like cracks, splitting of corrosion products on some areas of the surface, on the boundary to the base metal, inside the corrosion product layer etc.
This contribution treats the effects of thermal stresses and metal oxidation on the occurrence and development of defects leading to failure. An analysis is made of a čase vvhere due to mechanical instability, the oxide surface coating broke, and through the created cracks the corrosion medium came into contact vvith the metal. On such places an intensified oxidation took plače. Due to the special way of transfer of the oxidizing agent through the broken oxide layer, on these places the oxides grevv in the form of vvedges. Corrosion products differ essentially from metals so by their physical as vvell as their mechanical properties. As a result the properties inside such a system are very non-uniform, so the stress-strain states vvhich occur at temperature changes, cause the deformation of the corrosion products, and change their geometric characteristics.
The results of a general analysis of the model repre-senting the discussed system are illustrated by three examp!es: an element of tool for die casting of brass, pipes of a steam superheater from a thermal povver plant, and anode rods of a car battery. These examples
*	Univerza Edvarda Kardelja v Ljubljani, FNT, VTOZD Montanistika ** Univerza Edvarda Kardelja v Ljubljani, Fakulteta za strojništvo
*	E. Kardelj University of Ljubljana, Faculty of Natural Sciences, Dept. of Metallur.
** E. Kardelj University of Ljubljana, Faculty of Mechanical Engineering
Visokotemperaturna oksidacija je eden od pogostih načinov korozije kovin (suha korozija). Povišane in visoke temperature in nihanje temperature še dodatno močno obremenjujejo kovino, zato so poškodbe v takih okoljih še bolj pogoste in usodne. Take pogoje bomo upoštevali pri nastanku in rasti oksidnega klina.
2. NASTANEK IN RAST OKSIDNEGA KLINA
Nastanek oksidnega klina sledi predhodni enakomerni oksidaciji površine kovine na primerno visoki temperaturi. Na površini nastali oksid ima različen temperaturni razteznostni koeficient od kovine. Prav tako je pomembno, da ima nastali korozijski produkt tudi znatno večji specifični volumen. Fizikalne in mehanske lastnosti oksida in kovine se s temperaturo spreminjajo. Na obravnavani pojav pa vpliva tudi trdnost vezi med oksidom in kovino.
2.1	Nastanek oksidnega klina
Obravnavali bomo primer, kjer sta nastanek in širjenje oksidnega klina možna zaradi menjajočih se temperaturnih obremenitev. Na primerno visoki temperaturi nastane na površini kovine v določenem času zvezna plast oksida. Pri ohlajanju sistema kovina — oksid se zaradi različnih temperaturnih razteznostnih koeficientov v oksidu pojavijo tlačne napetosti. Zaradi njih se pri višjih temperaturah kovina in oksid plastificirata; ko pa se temperatura zniža in preide sistem iz plastičnega v elastično območje, začno v oksidu naraščati tlačne napetosti. Oksidni sloj na površini se elastično deformira. Z ohlajanjem lahko tlačne napetosti v oksidu dosežejo mejo plastičnosti. Če je trdnost vezi s kovino dovolj velika, prične oksid ponovno plastično teči. Drugače pa se lahko lokalno ukloni ali pa lušči, če je zrušilna stri-žna trdnost na meji s kovino manjša od tangencialnih napetosti. Ponavadi potekata oba pojava istočasno.
Pri ponovnem segrevanju sistema se pri določeni temperaturi eventuelne preostale tlačne napetosti v oksidu izničijo zaradi različnega širjenja kovine in oksida. Nato se s segrevanjem v oksidu pojavi in narašča natezna napetost. Zaradi mehanskih poškodb, ki so nastale med ohlajanjem, se v oksidu pojavijo koncentracije napetosti. Na teh mestih se pri nadaljnem segrevanju še razmeroma krhek oksid lahko poruši z razpoko, ki je pravokotna na površino kovine. Nastane lahko več takih razpok. Te razpoke omogočajo prost in hiter dostop zraka do kovine, zaradi česar pride do omejene, lokalne oksidacije. Ta drobna oksidna zajeda je začetek oz. zarodek oksidnega klina.
2.2	Rast oksidnega klina
Nastali oksid ima znatno večjo prostornino od kovine. Volumska deformacija zaradi oksidacije je tolikšna, da bi napetostno stanje daleč preseglo porušno trdnost kovine in oksida. Zato se oba (sistem) plastificirata. Napetostno stanje je v oksidni zajedi (zarodku klina) enako manjši meji tečenja ene od obeh sestavin sistema OTmin, kot je:
CTTmin = min (CT?*in, a^mm)
Pri ohlajanjih s Tmax bi se v oksidu pojavile tlačne napetosti, zaradi katerih sistem plastično teče. Plastično tečenje poteka vse do temperature prehoda sistema v elastično stanje pri Tp (temperatura prehoda sistema v
should point to the relative frequency of the phenomen-on. In literature very few references can be found about the phenomenon of an oxide wedge. The effect of the oxide vvedge on crack propagation was analysed by P. T. Heald (1) who proved that a crack cannot start grovving unstably only because of an oxide vvedge if no load is applied. Some investigations about the import-ance of an oxide vvedge for the development of defects on various machine parts can also be found in our pro-fessional literature2 3 5 6.
High temperature corrosion is one of the most fre-quently found types of metal corrosion (dry corrosion). High temperatures, raised temperatures and high temperature cycles represent an additional load for metal, making the deffects in these environments even more frequent and fatal. These very conditions will be consi-dered in our investigation of the occurrence and grovvth of an oxide vvedge.
2. OCCURRENCE AND GROVVTH OF OXIDE VVEDGE
And oxide vvedge occurs after a previous uniform oxidation of a metal surface at a correspondingly high temperature. The newly formed oxide has a different thermal expansion coefficient than the metal. It is also important that this nevv corrosion product has a consid-erably larger specific volume than the metal. The physi-cal and mechanical properties of the oxide and the metal are changing vvith temperature. Besides this, the dis-cussed phenomenon is affected also by the strenght of the oxide adhesion to the metal.
2.1. Oxide Wedge Occurrence
We vvill discuss a čase vvhere the occurrence and grovvth of the oxide vvedge are possible because of changing thermal loads. At a correspondingly high temperature and vvithin a certain tirne a continuous oxide layer occurs on the metal surface. During the cooling process of the metal/oxide system compressive stresses arise in the oxide due to the different thermal expansion coefficients. At higher temperatures the oxide and the metal become plastic because of the compressive stresses, but vvhen the temperature lovv-ers, and the system passes from the plastic into elastic region, the compressive stresses in the oxide start in-creasing. Now, the oxide layer on the surface undergoes elastic deformation. During the process of cooling the compressive stresses in the oxide can reach the plastic limit. If the strength of the adhesion to the metal is strong enough, the oxide starts flovving plastically. Othen/vise, it can bend locally or split in čase that the breaking shear strength on the metal boundary is smaller than the tangential stresses. Generally, hovvever, both these tvvo processes are going on simultaneously.
During reheating of the system, at a certain temperature the possible remaining compressive stresses in the oxide become eliminated because of the different ex-pansion of the metal and the oxide. If heating is continu-ed, tensile stresses appear and increase in the oxide. Due to mechanical defects that occurred in the oxide during the process of cooling, different stress concen-tration areas can be found. With continued heating on these areas, a relatively brittle oxide can break vvith a crack running rectangularly to the metal surface. Several such cracks might occur. These cracks enable a free and tast transfer of the oxidant (air) to the metal, indu-
elastično stanje pri tlaku). Napetostno stanje v oksidni zajedi je pri tej temperaturi enako:
Ormin (Tp = amin (a°K (Tp), a? (Tp»
Z ohlajenjem pa tlačne napetosti naraščajo. Če dosežejo mejo tečenja aTmin, pride do ponovnega plastičnega tečenja sistema.
Naslednja stopnja v spremenljivem temperaturnem režimu je segrevanje sistema od Tmin na Tm?x. Pri tem se v začetku tlačne napetosti, nastale pri ohlajanju, zmanjšujejo in pri določeni temperaturi je sistem brez napetosti. S segrevanjem se v zajedi pojavijo natezne napetosti, ki s temperaturo rastejo. Če napetost preseže trdnost zajede pod temperaturo prehoda iz elastičnega v plastično področje (T+ — pri natezni obremenitvi), se zajeda poruši. Razpoka je nadaljevanje razpoke v zvezni površinski plasti oksida. S tem se ponovno odpre hitra pot za dostop oksidanta do kovine ter se tako pospeši rast zajede v smeri razpoke. Po večkratni ponovitvi temperaturnega cikla (Tmax-Tmin-Tmax) se zajeda izoblikuje v obliko klina mikroskopskih razsežnosti.
3. NAPETOSTI ZARADI OKSIDNEGA KLINA
Rast oksidnega klina v kovini uravnava predvsem hiter prenos kisika po razpoki do kovine, v smeri normalno na steno klina, pa difuzija skozi oksid. Na ta način nastane in se ohranja trikotna oblika klina. Vsaka razpoka skozi primarni oksidni sloj je lahko začetek oksidne zajede oz. klina. Zato je na kovinskih delih, ki so izpostavljeni menjajočim se temperaturam, veliko mikroskopskih poškodb v obliki oksidnih klinov.
Napetostno stanje, ki se pojavi v sistemu med rastjo oksidnega klina, raziskujemo na poenostavljenem modelu, tako da izberemo tanek sloj polprostora, ki ga predstavlja polravnina z več oksidnimi klini. Napetostno stanje je odvisno od temperaturnih obremenitev in ga bomo analizirali pri treh značilnih mejnih temperaturah.
3.1 Napetostno stanje pri najvišji temperturi, Tmgx.
Pri tej temperaturi oksidni klin najhitreje raste. V njem se pojavijo tlačne napetosti, ki dosežejo minimalno mejo tečenja ene od sestavin sistema oTmin. Ker pa se oksid in kovina razlikujeta tudi v drugih lastnostih, se pojavijo še dodatne temperaturne napetosti. Te napetosti so v temenu klina, to je na meji polravnine razmeroma velike tlačne napetosti in so vzporedne z mejo polravnine. Pod mejo polravnine delujejo na klin znatne strižne in normalne napetosti pravokotno na smer polravnine5. Primerjalna napetost v sistemu kovina-oksid je znatno večja od meje tečenja, zato je hitrost plastične deformacije velika in napetostno stanje ne preseže aTmin. Rezultanta sil zaradi tlačne napetosti na plašču klina (aTmin) deluje proti meji polravnine. Zaradi nje in temperaturnih napetosti se sistem značilno deformira. Teme oksidnega klina se pri tem zoži, kovina pa s plastičnim tečenjem zavzame ta prostor. Posledica re-zultirajočih napetosti je izbočitev meje polravnine okoli temena oksidnega klina (si. 1). Zelo izraziti primeri plastične deformacije sistema so takrat, ko na temenih oksidnih klinov izpade del oksida in se poruši ravnotežno napetostno stanje na tem delu polravnine (si. 2, 3).
Oksidni klin vpliva na stabilno oz. nestabilno rast razpoke v kovini zaradi tlačnih napetosti, ki so enake meji tečenja oTmin (Tmax). Dejanska širina korena razpoke je (2):
cing a limited local oxidation. This tiny oxide flaw repre-sents the beginning of an oxide wedge.
2.2. Oxide Wedge Growth
The newly formed oxide has a much larger volume than the metal. The volume deformation due to oxidation is so extensive that the stress state exceeds by far the rupture strength of the metal and the oxide. Therefore both of them (the metal/oxide system) become plastic. The stress state in the oxide flaw (vvedge embryo) is equal to the lower yield point of one of the system's components as follovvs:
aymin = arnin (ay°*in, ayMmln)	(1)
During cooling from Tmax, compressive stresses ap-pear in the oxide due to vvhich the system exceeds the yield point. The plastic flovv continues down to the temperature of the transition of the system from the plastic into elastic state, Tp (temperature of the system's transition into elastic state under pressure). The stress state in the oxide flavv at this temperature is:
ay min (TP) = o min (a°K (Tp), ayM (Tp))	(2)
With continued cooling the compressive stresses in-crease. If they attain the yield point aymin, this induces a repeated plastic flovv of the system.
In changing temperature conditions, the next stage is heating up the system from Tmin onto Tmax. Here, at the beginning, the compressive stresses induced by cooling, are reduced and at a certain temperature the sys-tem is free of stress. With heating up the system, tensile stresses appear in the flavv, increasing vvith temperature. If the stress exceeds the strength of the flavv belovv the temperature of the transition from the elastic into plastic state (T+ — under tensile load), the flavv breaks. The newly occurred crack continues the crack in the primary surface layer of the oxide. In this way, the oxidant has again a fast access to the metal reopened, stimulating the grovvth of the flavv in the direction of the crack. After several repetitions of the temperature cycle (Tmax — Tmjn — Tn,ax) i the flavv grovvs into a vvedge of micro-scopic dimensions.
3. STRESSES INDUCED BY OXIDE VVEDGES
The grovvth of the oxide vvedge into the metal de-pends especially on hovv rapid is the access of the oxi-dant to the metal along the crack, and in the direction normally to the vvedge face, on hovv strong is the diffusion through the oxide. In this way the vvedge grovvs in the form of a triangle and keeps this form. Any crack through the primary oxide layer can mean the onset of an oxide flavv or vvedge. As a result, on metal parts vvhich are exposed to changing temperature conditions, a great number of microscopic defects in the form of oxide vvedges can be found.
The stress state occurring in the system during the process of oxide vvedge development, is investigated on a simplified model by choosing a thin layer of semi-space, representing the semi-plane vvith several oxide vvedges. The stress state as function of thermal loads will be analysed only for three extreme temperatures.
3.1. Stress State at the Highest Temperture (Tmax)
At this temperature the oxide vvedge grovvs the most rapidly. Compressive stresses appear in it, attaining the
Slika 3
Deformacija kovine in oksidnega klina zaradi odluščenja oksida na temenu klina; 40 x Fig. 3
Deformation of the metal in the oxide vvedge due to oxide split-ting on the vvedge back face; 40 x
minimum yield point of one of the system's components aTmin. But since the oxide and the metal differ also in other properties, additional thermal stresses appear too. On the back face of the oxide vvedge, i. e. on the semi-plane boundary, these stresses are compressive, and relatively high, acting in the direction parallel to the se-mi-plane. Under the semi-plane boundary considerable shear and normal stresses act on the vvedge in the direction rectangular to the semi-plane (5). The compara-tive stress in the metal-oxide system is considerably higher than the yield point, therefore, the rate of plastic deformation is high, and the stress state does not ex-ceed the aymin. The resultant of the forces arising from compressive stress on the vvedge faces (aymin), acts in the direction tovvards the semi-plane boundary. Because of this and due to temperature stresses, the system un-dergoes a typical deformation.
The back face of the oxide vvedge narrovvs, its plače being taken by the plastically flovving metal. The conse-quence of the stresses resulting from this is the buck-ling of the semi-plane boundary around the oxide vvedge, (Fig. 1). Very distinct cases of plastic deformation of the system can be observed if on the back face of the vvedge a tiny piece of the oxide splits off and de-stroys the equilibrium stress-state in this part of the semi-plane (Fig. 2, 3).
The oxide vvedge has an influence on the stable or unstable crack grovvth in the metal ovving to compressive stresses being equal to the yield point oymin (Tmax).
The actual vvidth of the crack tip is (2)
tan 0 + ti oy min (Tmax)
.1 -v
(3)
fl(c)= c(1~v) 4 7t |x au (Tn
vvhere 0 is the vvedge angle of the crack in the metal, v Poisson's number and n shear module.
And the critical vvidth of the crack tip at unstable grovvth is: <pc = 2y/o2r (Tmax) vvhere y is the surface stress. Crack grovvth is stable if (pc># (c) and unstable and leading to failure if QC<Q (c).
3.2. Stress State at the Transition Temperature from the Plastic into Elastic State (Tp)
In this čase the compressive stresses in the oxide vvedge are increased, hovvever not beyond the yield
tan 0 + ji atmin
kjer je 0 kot klina razpoke v kovini, v Poissonovo število in n. strižni modul.
Kritična širina korena razpoke pri nestabilni razpoki pa je: tpc = 2y/ozr (Tmax), kjer je y površinska napetost. Razpoka je stabilna, če je (pc>cp(C). Če pa je <pc<(p (C), je nestabilna in vodi k porušitvi.
<p(Q=	^
4n u a„ (Tm„)
Slika 1
Razpoka skozi oksidno plast in klin do kovine na vrhu klina. Fig. 1
A crack running through the oxide layer and the vvedge right to the metal at the vvedge tip
Slika 2
Deformacija kovine v okolici klina. Razpoke v smeri osi klina; 50 x
Fig. 2
Deformation of the metal in the vicinity of the vvedge. Cracks in the direction of the vvedge axis; 50 x
3.2 Napetostno stanje pri temperaturi prehoda iz plastičnega v elastično področje sistema, Tp
Tlačne napetosti v oksidnem klinu se povečajo, vendar ne čez mejo tečenja. V tem primeru lahko določimo največje napetostno stanje v kovini ob korenu oksidnega klina, prav tako pa faktor koncentracije napetosti4. Napetost v oksidnem klinu je enaka oTmin (Tp), faktor koncentracije napetosti pa: K,= l,1209-oTmin (Tp) j/č, kjer je c dolžina oksidnega klina. Če pa je na površini več klinov, je faktor koncentracije napetosti v kovini ob
Slika 4
Skupine oksidnih klinov; 50 x Fig. 4
Groups of oxide vvedges; 50 x
korenih klinov manjši (si. 4, 5). Če je v polravnini več klinov dolžine c, ki so med sabo oddaljeni z d, je faktor koncentracije napetosti K| = proTmin (Tp) j/č.
X, 0	0,2	0,4	0,6
Pi 1,1209 0,87186 0,62536 0,51046
0,8	1,0	2,0	3,0
0,4446 0,39866 0,282206 0,2303
kjer je X. = c/d.
Sistem z več enako velikimi oksidnimi klini v polravnini je odpornejši proti porušitvi v primerjavi s sistemom, ki ima le enega samega iste dolžine.
point. We can define the maximum stress state as well as the stress concentration factor in the metal around the oxide vvedge tip4. The stress in the oxide vvedge is equal to aTmin (Tp) and the stress concentration factor is K, = 1,1209-a-rmin (Tp) ^c, vvhere c represents the length of the oxide vvedge. In čase of several vvedges on the surface, the stress concentration factor in the metal around the vvedge tips is smaller (Fig. 4, 5). If there are several vvedges in the semi-plane of various lengths (c) and distances (d), then the stress concentration factor is K, = P,-CTTmin (Tp) yč.
X	0	0,2	0,4	0,6
P,	1,209 0,87186 0,62535 0,51046
0,8	1,0	2,0	3,0
0,4446 0,39866 0,28206 0,2303
Slika 5
Shema sistema kovina-oksidni klini. Fig. 5
Scheme of the metal/oxide vvedge system
3.3 Napetostno stanje pri najnižji temperaturi, Tmin
Tlačne napetosti v oksidnem klinu se povečujejo in dosežejo pri neki Tmin vrednost, ki presega napetostno stanje pri vseh višjih temperaturah. Faktor koncentracije napetosti se zato poveča in je določen na enak način kot zgoraj.
Tako kot pri najvišji, je tudi pri najnižji temperaturi zanimiv odgovor na vprašanje o stabilnosti razpoke v kovini. Hitrost širjenja razpoke č (t) je:
Ji(l-v)l
-0,5
) ta (t) q
2 Y .
-1
Odtod sledi pogoj za nestabilno rast razpoke: co = 2 y/a, kjer je co širina temena oksidnega klina. V tem primeru je hitrost širjenja razpoke neskončno velika.
4. MOTNJE V RASTI OKSIDNEGA KLINA
Motnje v rasti oksidnega klina se pojavijo takrat, ko se v določenih razmakih ne obnavlja razpoka v oksidni plasti ali klinu kot hitra pot za prenos kisika do kovine na vrhu klina. V nekaterih primerih se lahko zgodi, da oksid zapre zunanjo stran razpoke, tako da se tudi pri nihanju temperature zapreka ne poškoduje oz. ne poči. Takrat je oksidacija vezana zgolj na transport skozi oksid. Rast klina se tedaj upočasni, posebej še na korenu, zato se mu spremeni tudi oblika. Zelo trdno zaporo
vvhere X=c/d
A system vvith several equally sized oxide vvedges in the semi-plane is more resistant to rupture than a sys-tem vvith only one vvedge of the same length.
3.3. Stress State at the Lovvest Temperature (Tmln)
The compressive stresses in the vvedge increase, and at a certain minimum temperature (Tmin), attain the value vvhich exceeds the stress state at ali higher temperatures. The stress concentration factor is therefore increased, and can be defined in the same way as above.
Similarly as for Tmax, vve are also for Tmin interesed in the ansvver to the question of stable or unstable crack grovvth.
The rate of crack propagation c (t) (2) is:
-0,5
cm—g*,
— v)
"(t) { -v) o l
1-
w(t) g
-1
(4)
2 Y
Therefrom the condition for the unstable crack grovvth is obtained: w = 2y/o vvhere w is the vvidth of the oxide vvedge back face. In this čase the rate of crack propagation is infinite.
4 RETARDATION IN GROVVTH OF AN OXIDE VVEDGE
Retardation in the grovvth of an oxide vvedge takes plače vvhen from tirne to tirne a crack enabling a fast
za dostop kisika predstavlja npr. kompozitna plast oksida in kovine, ki lahko nastane v določenih delovnih okoljih oz. pogojih. Ta je odporna na menjajoče se temperaturne obremenitve, posebej v fazi ogrevanja (nateg) in je vzrok dolgotrajni motnji v rasti oksidnega klina. Taka kompozitna zapora je manj odporna na tlačne oz. strižne obremenitve, zaradi katerih se lahko odlušči s površine in motnja preneha (si. 6, 7, 8).
Slika 6
Mehanske stabilne pregrade (komposit oksid-kovina) nad oksidnim klinom. Fig. 6
Stable mechanical closure barrier (an oxide/metal composite) above the oxide wedge
Slika 7
Mehanske stabilne pregrade (komposit kovina-oksid) nad dvema degeneriranima oksidnima klinoma; 50x Fig. 7
Stable mechanical barrier (an oxide/metal composite) above two degenerated oxide vvedges; 50 x
transfer of oxygen to the metal does not reopen. In some cases it can happen that corrosion products close the crack from the outside so that even at varying temperature this closure does not break. In these cases oxi-dation depends only on the oxygen transport through the oxide. Ovving to this, the growth of the vvedge slows down especially at the tip, thus changing also the vvedge morphology. A very solid closure for the transfer of oxy-gen represents for example a composite layer of oxide and metal created in specific vvorking conditions. This layer is resistant to changing temperature loads, espe-cially in the phase of heating (tension), and this is the reason for a long retardation in the oxide vvedge grovvth. Such a composite closure can, hovvever, be less resistant to compressive and shear loads due to vvhich it can peel off the surface, and the vvedge starts grovving again (Fig. 6, 7, 8).
Slika 8
Kemična sestava oksidnega klina in stabilne pregrade. Fig. 8
Chemical composition of the oxide vvedge and the stable barrier
5. SOME EXAMPLESOFOXIDE VVEDGE OCCURRENCE
5. PRIMERI NASTAJANJA OKSIDNIH KLINOV
5.1
Bat stroja za tlačno litje medi je izdelan iz orodnega jekla za delo v vročem (0,4% C, 5 % Cr, 1,3 % Mo in 0,4 % V)5. V stacionarnih pogojih dela je nihala temperatura na površini bata v približno 11 sekundah od 780 °C (Tmax) do 600 °C (Tmin), ob prekinitvah pa se površina bata ohladi pod 200 "C ali celo na temperaturo okolice. Po določenem času dela se je bat poškodoval zaradi t. i. toplotnega razpokanja na delovni površini. Te poškodbe se kažejo v mreži bolj ali manj globokih razpok-kanalov, ki se širijo v kovino v obliki oksidnih klinov. Na osnih presekih bata je tako moč opaziti
5.1.
The plunger of a die casting machine for brass is made from hot-vvorking tool steel (0.4% C, 5% Cr, 1.3 % Mo and 0.4% V) (5). At stable operating conditions the temperature on the plunger surface varies in approx. 11 seconds from 780° C (Tmax) to 600° C, vvhile at vvork stoppage, the plunger surface cools dovvn belovv 200° C, or even dovvn to the ambient temperature (Tmin). After a certain tirne of operation, heat cracking occurs on the vvorking surface of the plunger. These defects can be seen.as a netvvork of more or less deep cracks — channels — extending into the metal in the form of oxide vvedges. On the axial cross section of the plunger it is thus possible to observe practically ali the above de-
praktično vse prej opisane pojave: razpoke v klinih, deformacije klinov in okolišnje kovine, zaprtje razpok in motnje v rasti oksidnega klina (si. 2, 3, 6, 7, 8).
5.2
Na zunanji steni cevi pregrevalnika pare, ki dela pri nižjem temperaturnem nivoju kot bat, so se pod relativno tanko plastjo škaje pojavili oksidni klini. V nekaterih primerih se je iz teh klinov razvila poškodba do porušitve stene cevi. Poleg agresivnega delovanja okolice (dimni plini) je sistem cevi podvržen tudi obremenitvam zaradi nihanja temperature (si. 9).
scribed phenomena: cracks in the wedges, deformation of vvedges and the surrounding metal, crack closure and retardation in the grovvth of the oxide vvedge etc. (Fig. 2, 3, 6, 7, 8).
5.2.
On the outer vvall of a steam superheater, operating at a lovver temperature level than the plunger, oxide vvedges vvere observed under a relatively thin layer of oxide scale. In some cases the defects arising from these vvedges developed into total failure of the pipe vvall. Besides the agressive effect of the environment (flue gases), the piping system is subjected also to loads arising from temperature variation (Fig. 9).
Slika 9
Oksidni klini z zunanje stene cevi pregrevalnika pare; 100x Fig. 9
Oxide vvedges from the outer side of the steam superheater tube; 100 x
5.3.
The same kind of defects in the form of corrosion product vvedges vvere found on positive electrodes of a car battery.
These defects resulted in local breaking of anode rods and total failure of the battery. The anode rods, vvhich are made of a low-alloyed Pb-alloy, are hung onto the battery boxing and thus constantly under ten-sion load. During battery charging and discharging a chemical reaction takes plače. Its products differ very much in specific volume, so the PbS04 probably cannot bear the vveight of the anode rod and breaks. The ex-changing cycles of discharing and charging at constant tensile load enable the grovvth of corrosion products in the form of vvedges (Fig. 10). When the crack reaches a certain critical point deep enough in the metal, the rod undergoes a sudden brittle failure.
5.3
Povsem enake poškodbe, v obliki klinov korozijskih produktov, so nastale na pozitivnih elektrodah akumulatorskih baterij in so pripeljale do lokalnih zlomov palic in uničenja baterije. Palice iz malolegirane svinčeve zlitine so obešene v bateriji in zato so ves čas zaradi lastne teže obremenjene na nateg. Pri polnjenju in praznjenju baterije poteka kemična reakcija, katere produkti se močno razlikujejo v gostoti. Trdnost sulfata PbS04 je manjša od napetosti zaradi teže palice in se poruši. Menjajoči se ciklusi praznjenja in polnjenja pri stalni natezni obremenitvi omogočajo rast korozijskega produkta v obliki klinov (si. 10). Ko seže poškodba zadosti globoko v kovino, se palica nenadno poruši (krhko).
6. ZAKLJUČEK
Kovinski deli orodij in naprav, ki delajo v agresivnih okoljih, se prekrijejo s plastmi korozijskih produktov. Zaradi mehanskih ali temperaturnih obremenitev so korozijski produkti pogosto mehansko nestabilni, kar pripelje do lokalnih porušitev. Skozi razpoke prihaja medij zelo hitro do kovine in na teh mestih začno rasti korozijski produkti v obliki klinov.
Če so izpolnjeni pogoji trajne mehanske nestabilnosti korozijskih produktov, klin raste in pripelje do porušitve istema. V sistemih s korozijskimi klini nastanejo značilna napetostno deformacijska stanja. Ta vplivajo na oblikovanje sistema in eventuelno porušitev! Mot-
Slika 10
Klin v anodni palici akumulatorske baterije; 100 x Fig. 10
A vvedge in the anode rod of a car battery; 100 x
6 CONCLUSION
Metal parts of tools and machines, operating in agressive environment, get covered vvith corrosion product layers. Due to mechanical and thermal loads corrosion products are subject to mechanical instability, lead-ing to local failures. Through cracks the oxidant rapidly reaches the metal and on these places corrosion products start grovving in the form of vvedges.
In the conditions of permanent mechanical instability of corrosion products, a vvedge grovvs and causes a fai-

nje, ki preprečujejo hiter dotok korozijskega medija do kovine na vrhu klinov, zavro njihovo rast in spremene njihovo obliko. Ugotovljeno je tudi, da je rast skupine oksidnih klinov v enakih pogojih počasnejša kot pa takrat, če je v sistemu en sam klin.
V	prispevku smo obravnavali rast klinov v kemično in mikrostrukturno homogenem kovinskem materialu.
V	kemično nehomogenih materialih so s potekom koncentracije legirnih elementov določena prednostna mesta nastanka in rasti klinov. Če je medsebojna orientacija nehomogenosti in komponent temperaturnih napetosti ugodna, potekajo razpoke oz. oksidni klini vzdolž negativnih izcej. Na teh mestih so razlike v razteznostnih koeficientih kovine in oksida največje; oksid nad negativno izcejo je med nihanjem temperature znatno bolj obremenjen kot nad pozitivno, zato večina razpok v oksidu nastane nad negativnimi izcejami. Analiza takega primera je zahtevnejša in bo vsebina samostojnega prispevka.
lure of the system. In corrosion vvedge systems very ty-pical stress-strain states occur, affecting the morpholo-gy of the system and inducing a possible failure. Clo-sures preventing a rapid access of the oxidant to the metal at the vvedge tip, retard the grovvth of the vvedge and change its morphology. The authors also found out that the grovvth of a group of oxide vvedges is slovver than that of only one vvedge in the system.
This contribution treats oxide vvedge grovvth in a ho-mogeneous metal material. In metal materials vvith non-homogeneous chemical composition and microstructure the places likely for the occurrence and grovvth of vvedges are defined from the concentration distribution of the alloying elements. If the interorientation of non-homogeneities and thermal load components is favor-able, cracks or vvedges run along the negative segrega-tions. On these places the differences in thermal expan-sion coefficients betvveen the metal and the oxide are the greatest; the oxide above the negative segregation is during temperature variation under much greater load than above the positive segregation, therefore the major part of cracks in the oxide are to be found above the negative segregations. The analysis of such a čase de-mands special attention and efforts, and vvill be the sub-ject of a separate investigation.
LITERATURA/REFERENCES
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