Korrosion hos zinkskikt

Stål är vår tids mest använda material. Tyvärr har stål en stor nackdel och det är dess höga korrosionshastighet. Att skydda detaljer och konstruktioner av stål har därför ett stort värde. Det bästa korrosionsskyddet erhålls med zink. Zinkbeläggningar skyddar stål på två sätt: • Genom barriärverkan, dvs hindrar syre och fukt att tränga fram till stålytan. • Genom att ge katodiskt skydd i repor, slagmärken, klippkanter osv. Zink är en oädel metall med stor korrosionsbenägenhet. Att korrosionshastigheten ändå är låg i de flesta miljöer beror på att zinkens yta snabbt blir täckt med korrosionsprodukter, som sedan skyddar mot vidare korrosion

Korrosion i atmosfären

När ett varmförzinkat föremål lämnar zinkbadet angrips ytan genast av luftens syre och zinkoxid bildas. Processen fortskrider med hjälp av luftens vatten- och koldioxidinnehåll varvid basiska zinkkarbonater skapas. Dessa ger ett tätt skikt med utmärkt vidhäftning. Då karbonaterna dessutom har mycket låg löslighet i vatten utgör de ett gott skydd för den förzinkade ytan. Men luften innehåller också svaveldioxider, som omvandlar det basiska zinkkarbonatet till mer lättlösligt zinksulfit och zinksulfat. Luftens halt av svaveldioxid har dock minskat kraftigt de senaste åren, och därmed också zinkens korrosion. Atmosfärens halt av svaveldioxid påverkar således korrosionshastigheten. I industriatmosfär är därför korrosionen högre än i stads- och landsbygdsmiljö. Av betydelse i alla miljöer är exponeringsvinkeln. Korrosionen är högre på en horisontell yta än på en vertikal yta. Ytor som befinner sig i lä korroderar mindre än oskyddade ytor.

Zinkskikt som exponeras några månader i atmosfären får en matt, ljusgrå kulör (bild 1). I havsmiljö påverkas zinkens korrosion av luftens salthalt. I den marina luften finns dock små halter av magnesiumsalter med god passiverande verkan. Korrosionen är därför inte så stor som man befarat. Salthalten avtar också snabbt in mot land.

Flera faktorer inverkar

Zinkens korrosion påverkas alltså av många faktorer. Det går därför inte att ge en allmängiltig formel för korrosionshastigheten. Zinkbeläggningar har emellertid använts under lång tid och under de mest skiftande förhållanden för att skydda stål mot rost. Kunskapen om zinkens korrosion och korrosionshastighet i olika miljöer är därför god. Vi har idag exempel på zinkbeläggningar som exponerats i över hundra år.

Rödbrun missfärgning

Kiseltätade stål som har stor andel järn-zinkfas i zinkskiktet kan efter en tids exponering få en rödbrun färgton som djupnar med tiden. Järn-zinkfasen korroderar, varvid järn frigörs, som tillsammans med fukt från luften eller regnvatten bildar rost. Rosten har stor färggivande förmåga och även små mängder kan ge avsevärda missfärgningar (bild 2) . En kraftig missfärgning kan lätt uppfattas som att rostskyddet är nedsatt eller helt slut. Detta är dock sällan fallet. Järnzink-legeringen skyddar stålet bättre än ren zink – upp till 30 % förbättring har noterats. Den missfärgade ytan kan, om så krävs av utseendeskäl, med fördel målas. En missfärgad lyktstolpe som exponerats i 30 år hade vid undersökning ca 70 mikrometer zinkskikt kvar – tillräckligt för ytterligare ca 50 års livslängd.

Bild 1. Exponerad yta av en zinkbeläggning med ett yttre skikt av ren zink. Den blanka ytan försvinner och ersätts av grå korrosionsprodukter – ibland kallat zinkpatina.

Bild 2. Rödbrun missfärgning

Korrosion i vätskor

Liksom vid korrosion i luft överdras zinkytan vid nedsänkning i en vätska i allmänhet med ett skyddande skikt av korrosionsprodukter. Vätskor kan vara sura eller alkaliska och innehålla lösta eller fasta aggressiva ämnen. Vätskans strömningshastighet och temperatur har också stor betydelse. Tillsammans medför dessa faktorer att skyddsskiktet kan få en högst varierande sammansättning eller t.o.m. inte bildas alls. Den elektrokemiska korrosionen, som i luft spelar en underordnad roll, har stor betydelse i vätskor. Beroende på vätskans elektriska ledningsförmåga sker den elektrokemiska korrosionen, och därmed också zinkskiktets skyddsverkan, över större eller mindre områden. Största betydelse har vätskans pH-värde. Zinkens korrosionshastighet är normalt relativt låg och stabil i pH-området 5,5-12,5 vid temperaturer mellan 0 och 20 °C.

Hårda vatten, som innehåller kalk och magnesium, är föga aggressiva. Dessa ämnen bildar tillsammans med kolsyra svårlösliga karbonater på zinkytan och ger stabila skyddsskikt, som hindrar vidare korrosion. Mjuka vatten angriper ofta zink eftersom salter saknas och skyddsskiktet inte kan bildas. I mindre vanliga fall kan det också ske en polaritetsväxling mellan zink och stål så att stål blir anod (lösningspol) i elementet. Risk för punktkorrosion finns i sådana fall. Polaritets växlingen motverkas av kolsyra, sulfater och klorider och sker därför inte i t ex havsvatten, men kan däremot förekomma i mycket rena vatten. Aggressiva mjuka vatten finns i vissa vattendrag och sjöar i Finland, Norge och Sverige.

Har vattnet högre strömningshastighet än 0,5 m/s hindras bildningen av skyddsskikt på zinkytan och korrosionen blir snabbare. I vatten har temperaturen stor betydelse för korrosionshastigheten. Över ca 55 °C får de skiktbildande korrosionsprodukterna en grovkornig struktur och förlorar vidhäftningen till zinkytan. De faller lätt av och blottlägger ny, frisk zink för fortsatt angrepp. Korrosionshastigheten når ett maximum vid ca 70 °C, för att sedan sjunka så att den vid 100 °C är av samma storleksordning som vid 50 °C. Som framgår är korrosionsförloppet i vatten mycket komplext och allmängiltiga regler är svåra att ge. Praktiska erfarenheter finns och bild 10-3 ger riktvärden för några olika typer av vatten.

Bild 3. Medellivslängd för olika tjocka zinkskikt i skilda vatten.


Wet-storage stain

Fig.4. Wet-storage stain.
Fig. 5. In order to avoid the formation of wet-storage stain on newly galvanized surfaces, profiled steel, beams and structures should be packed at an angle and turned to prevent the accumulation of water

Vitrost Ibland uppträder en vit, mjölig och voluminös beläggning kallad vitrost eller vitblemma på förzinkade ytor (bild 10-4). Vitrost uppträder om förzinkat gods utsätts för kondens eller regnvatten som blir liggande kvar på godsytan. Detta kan inträffa där spalter bildas, t ex mellan hoplagda godsytor eller mellan tätt packade plåtar. Godsytor, som har god luftväxling och byggt upp ett normalt skyddsskikt av korrosionsprodukter, angrips sällan. Angreppet upphör således när fri tillgång till luft finns. Kvarvarande vitrost nöts så småningom bort av väder och vind. Zinkytan får åter ett för varmförzinkning normalt utseende.

The wet-storage stain layer is voluminous and porous and only loosely attached to the zinc surface. As a result, protection against continued attack does not exist. Corrosion can therefore continue as long as moisture remains on the surfaces. When wet-storage stain has occurred, the object should be stacked to enable the surfaces to dry quickly. This will stop the attack and, with free access to air, the normal protective layer will be formed. The wet-storage stain is gradually washed away and the coating acquires an appearance that is normal for exposed, hot-dip galvanized steel.

Since this white corrosion product is very bulky (about 500 times that of the zinc from which it was formed), it can appear to be serious. However, wet-storage stain often has little or no significance on the service life of the corrosion protection. In the case of very thin coatings however, e.g. on electroplated objects, a severe attack of wet-storage stain can be of significance.

Wet-storage stain is best avoided by preventing newly galvanized surfaces from coming into contact with rain or condensate during transportation. Materials stored outdoors should be stacked so that water can run off easily and so that all surf­aces are well ventilated (fig. 5). Painting after galvanizing gives very good protection.

Corrosion in soil

The corrosion conditions in soil are very complicated and variations can be great between different locations, even those in close proximity to each other. Soil can contain weathered products, free or bound salts, acids and alkalis, mixtures of organic substances, oxidizing or reducing fungi, micro- organisms etc. Depending on its structure, soil has different degrees of permeability to air and moisture. Normally, the oxygen content is less than in air, while the carbon dioxide content is higher.

Swedish soils are generally not very aggressive. Average corrosion for zinc is usually taken as 5 µm per year. Severely aggressive soils are seldom seen. In north and west Bothnia, soils may contain sulphur. They are often black, but lighten when exposed to air. In these soils, zinc corrosion rates are very high.
One method of determining the corrosiveness of a soil is to measure its resistivity. If the resitivity of the soil cannot be determined, the rules-of-thumb listed in figure 6 can give a measure of guidance. Where the exposure of metals to soil is concerned, however, it is advisable to seek expert advice from suitably qualified sources.

Fig. 6. Corrosiveness of different soil types.

Galvanic corrosion

Fig. 7. Electrochemical potential scale in sea water at + 25 °C

If two different metals or alloys, completely or partially surrounded by an electrolyte, are connected a galvanic cell is created. Which metal becomes the anode or cathode is determined by their electrode potentials in the electrolyte in question. In sea water, which corresponds to the majority of practical conditions, some metals and alloys take up different positions on the electrochemical scale, shown in figure 7.

If steel is connected to copper or brass, the steel becomes the anode in the cell and corrodes. However, if steel is connected to cadmium, aluminium, zinc or magnesium, it becomes the cathode and is protected against corrosion, while the anode metal is consumed. Galvanic corrosion is also called bi­metallic corrosion and is used to protect underwater structures from corrosion, when it is termed cathodic protection.

Cathodic protection afforded by zinc coatings

In hot-dip galvanized steel, zinc and steel are in good electrical contact with each other. If the zinc coating is damaged in the presence of an electrolyte a galvanic cell is created. The electrolyte could be condensate or rain water. Sometimes the entire structure can be submerged in liquid. In this cell the zinc becomes the anode or dissolving pole, the exposed steel becomes the cathode and is therefore protected from corrosion.

In the initial phase it is often possible to see a weak ust formation on the exposed part of the steel surface where the coating has been damaged, but after a while whitish-grey areas form which gradually spread over the entire damaged area. The zinc coating corrodes and sparingly soluble zinc alloys descend to the cathode surface where they protect the steel from continued rust attack. This is often called “self-healing”, which is something of a misnomer since the zinc layer is, of course, not restored.

Owing to the cathodic protection generated by the zinc, rust cannot “creep in” under the coating at the point of damage in the way that it can creep under films of paint or coatings of metals more noble than steel. Zinc coatings on steel are unusual, since a fairly large area of damage to the coating does not cause catastrophic loss of rust protection. The range of cathodic protection is dependent on the nature of the electrolyte that creates the cell. For structures in normal atmospheres it is usual to expect protective action over several millimetres. However, in sea water significantly greater distances can be expected.

Fig.8. Schematic picture of damage for different rust-prevention coatings.

Zinc coatings in contact with non-ferrous metals

The Electrochemical potential scale shows that zinc is less noble than most common metals. This means that when zinc is connected to these metals in a galvanic cell it is the zinc that becomes the dissolving pole. In principle, therefore, such connections should be avoided when possible. A good method is to use an insulator such as plastic or rubber at the joint.

Aluminium and stainless steel can often be connected directly to galvanized material in air or fairly dry environments without noticeable corrosion taking place. However, in water an insulator should always be used.

Copper and copper alloys are more electrically active and there is often a release of copper ions which spread over large surfaces and cause noticeable attack. For this reason, these metals should never be allowed to come into contact with galvanized steel and an insulator should be used.

Hot-dip galvanized steel in contact with building materials

Mortar, plaster and wood

Damp mortar and plaster attack zinc. The attack ceases when the material dries out. Dry or moderately damp wood, both impregnated and unimpregnated, can be nailed with hot-dip galvanized nails to good effect. However, in the case of nails or threaded unions that are constantly exposed to water an acid-resistant material is preferred. Other dry building materials, such as mineral wool, do not attack zinc.

Concrete

Reinforced concrete is an important construction material. Highway bridges, car parks, houses, office buildings, gulleys are examples of reinforced concrete structures. In many cases, its steel reinforcement does not need any external corrosion protection to avoid corrosion. The highly alkaline environment of concrete forms a thin oxide layer on the steel surface which protects or “passivates” the steel against further corrosion.

But, in some instances that passivation does not work well enough, or even not at all. This may occur when:

• The concrete has cracks, clevages, sand pockets or too little cover.
• The alkaline environment has been neutralized (carbonation).
• Chlorides have penetrated the concrete (marine environment, winter prepared roads).

These situations often occur in the outermost parts of a construction. Defects in reinforced concrete have increased because of increased salting and atmospheric impurities. Defects in different concrete constructions are more common today than previously recognised (fig. 9) . Once corrosion of the reinforcement has started it is very difficult and expensive to repair. The need to protect reinforcement from corrosion is becoming increasingly important in several concrete applications.

Fig. 9. Spalling of cover layer over reinforcement in basement, due to the corrosion of reinforcement bars.
Fig.10. Hot dip galvanized reinforcement in a bridge .

Corrosion protection for reinforcing steel

The possibility to protect reinforcing steel with hot-dip galvanizing is well documented by case histories in many countries. Many slender constructions use hot-dip galvanized reinforcement to avoid spalling which leads to expensive repair. It is also worth noting that concrete debris that may fall fom a cracked surface may cause serious harm, especially in urban areas.

Designers have been known to request that the section of a steel construction that is to be set into concrete should be free from zinc. This request is totally unnecessary and to prevent one part of a construction from zinc coating during galvanizing is often more expensive than galvanizing the whole construction. Designers should reassured that the adherence between the hot-dip galvanized surface and the concrete is normally so strong that a sledge hammer is needed to separate them.

Zinc has been used as sacrificial anode to protect ships’ hulls, harbour constructions, cisterns and similar structures against corrosion. Of the available metallic coatings hot-dip galvanizing has been shown to be the most durable and technically suitable. Hot-dip galvanizing of reinforcement steel for concrete has been used worldwide for many years. Even in very severe conditions this surface treatment has shown to be a reliable choice.

Detailed studies, for example in Australia and at KIMAB (Corrosion- and Metals Research Institute) in Sweden have shown the following results:

• Accelerated corrosion only takes place during at maximum the first 36 hours after pouring the concrete. After that, the coating is passivated. The loss of zinc is low, in the range 2-5 µm.

• Zinc gives cathodic protection on exposed steel surfaces, which is a benefit when cutting or welding the reinforcement or when it is mechanically damaged.

• The adherence between reinforcement steel and the concrete is good.

• Concrete spalling does not occur.

• The risk for discolouration of the facade due to rust runs is eliminated.

• By galvanizing it is possible to use reinforced concrete in more aggressive environments

• Variations in concrete quality are reduced.

• Thinner concrete cover can be allowed.


Hot-dip galvanized reinforcement in chloride environments

Practical experiments performed by KIMAB have shown that zinc copes very well even in chloride-containing environments. Up to 1.5 wt. % chloride in the concrete leads to negligible corrosion of the galvanized rebar. Conversely, unprotected reinforced steel had difficulty to cope with this chloride concentration and was corroded. Zinc can withstand even higher chloride concentrations, but with a related decrease in coating life. In such an environment unprotected steel in addition to normal corrosion also will show pitting corrosion, which does not occur on galvanized steel. Also in carbonated concrete, galvanized steel is more durable than unprotected steel.

Hot dip galvanized reinforcement is a reliable partner in concrete technology. It minimizes the risk of steel corrosion and concrete spalling and gives a strong and cost effective contribution to the durability of the concrete.

When cost and other consequences of corrosion damage of a construction are analyzed, the extra cost for galvanizing is negligible. It could almost be considered as an insurance premium – which only has to be paid once.

Even when the price of galvanized reinforcement may be up to 50 % more than black steel, it is still a negligible part of the overall budget for a construction.

Packing and transportation of galvanized steel

Even though a hot-dip galvanized coating is capable of withstanding fairly rough treatment it should be handled with care during storage and transportation. In case of long goods simple packing and binding into bundles not only protects against mechanical damage but it often facilitates transportation itself. However, packing and binding should be done in such a way as to avoid risk of wet storage strain. Spacers should be used to prevent such attacks.

For members
Svenska