Initially,the substrate is protected againstcorrosionbythe barrier action of
the coating, followed by the barrier action of the corrosionproduct layer.
Upon exposure to air,aluminum formsachemically inert Al 2 O 3 oxide film
that is ar apidly forming self-healing film. Therefore, the passivefi lm on
aluminum, as well as the corrosionproduct layer,isamainbarrier and leads
to ar esistant material in natural environments.
On the other hand, the surfaceo xide film that forms on zinc is not as an
effective barrierast he oxide film on aluminum.
Upon exposure in the natural atmosphere, many corrosion cells are
formed on the surfaceofasacrificial metal coating, thereby,accelerating the
corrosion rate. Duringthis period, corrosion products are gradually formed
and converted to as table layer.T his period may last for several months,
after this period, the corrosion rate becomesc onstant. These corrosion
products form the second barrier and are amorphous Al 2 O 3 on aluminum,
andZ n(OH) 2 andb asic zinc saltso nz inc. ZnO, being electrically
conductive, loosens the corrosion product layer and, therefore, does not
contribute to formation of the barrier.W hen materials such as CO 2 ,N aCl,
and SO x are present, basic zincs alts are formed, for example, 2ZnCO 3 $
3Zn(OH) 2 in mild atmospheres, ZnCl 2 $ 6Zn(OH) 2 in chloride atmospheres,
and ZnSO 4 $ 4Zn(OH) 2 in SO x atmospheres. How stablee ach basic zinc
salt will be is dependent on the pH and anion concentration of the
electrolyte on the zinc. Zinc carbonate formsa ne ffective barrier on steel in
mild atmospheres, whereas, basic zinc sulfate andc hloride dissolve with
decreasing pH of the electrolyte. The basic zincs ulfate is restricted under
atmospheric conditions in its effort to act as ab arrier since the pH value of
rain in an SO x atmospherei su sually low,i nt he area of less than 5. In a
chloridee nvironment, the value of pH in the electrolyte is not as low as in
the SO x atmosphere; therefore, asecondary barrier will form. However, in a
severe chloride environment, the zinc coating layer willc orrode in spite of
the existence of basic zinc chloride on the surface.
7.1.4.1 Galvanic Action of Coating Layer
Sacrificial metal coatings protect the substrate metal by means of galvanic
action. When the base metal is exposed to the atmosphere as ar esult of
mechanical damage or the like, the exposed portion of the basem etal is
polarizedc athodically to the corrosion potential of the coating layer.A sa
result, little corrosiontakes place on the exposed metal. Agalvanic couple is
formed between the exposed part of the base metal and the surrounding
coating metal. Sacrificial metals are more negativei ne lectrochemical
potential than other metals such as iron or steel. Therefore, the sacrificial
metal coating acts as ac athode. Thist ype of reaction of sacrificial metal
coatings is known as galvanic or cathodic protection. In addition,the defects
are protected by as econd barrier of corrosion products of the coating
layer.F igure 7.4s chematically illustrates the galvanica ctiono fasacrificial
metal coating.
Coatings
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CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
the coating, followed by the barrier action of the corrosionproduct layer.
Upon exposure to air,aluminum formsachemically inert Al 2 O 3 oxide film
that is ar apidly forming self-healing film. Therefore, the passivefi lm on
aluminum, as well as the corrosionproduct layer,isamainbarrier and leads
to ar esistant material in natural environments.
On the other hand, the surfaceo xide film that forms on zinc is not as an
effective barrierast he oxide film on aluminum.
Upon exposure in the natural atmosphere, many corrosion cells are
formed on the surfaceofasacrificial metal coating, thereby,accelerating the
corrosion rate. Duringthis period, corrosion products are gradually formed
and converted to as table layer.T his period may last for several months,
after this period, the corrosion rate becomesc onstant. These corrosion
products form the second barrier and are amorphous Al 2 O 3 on aluminum,
andZ n(OH) 2 andb asic zinc saltso nz inc. ZnO, being electrically
conductive, loosens the corrosion product layer and, therefore, does not
contribute to formation of the barrier.W hen materials such as CO 2 ,N aCl,
and SO x are present, basic zincs alts are formed, for example, 2ZnCO 3 $
3Zn(OH) 2 in mild atmospheres, ZnCl 2 $ 6Zn(OH) 2 in chloride atmospheres,
and ZnSO 4 $ 4Zn(OH) 2 in SO x atmospheres. How stablee ach basic zinc
salt will be is dependent on the pH and anion concentration of the
electrolyte on the zinc. Zinc carbonate formsa ne ffective barrier on steel in
mild atmospheres, whereas, basic zinc sulfate andc hloride dissolve with
decreasing pH of the electrolyte. The basic zincs ulfate is restricted under
atmospheric conditions in its effort to act as ab arrier since the pH value of
rain in an SO x atmospherei su sually low,i nt he area of less than 5. In a
chloridee nvironment, the value of pH in the electrolyte is not as low as in
the SO x atmosphere; therefore, asecondary barrier will form. However, in a
severe chloride environment, the zinc coating layer willc orrode in spite of
the existence of basic zinc chloride on the surface.
7.1.4.1 Galvanic Action of Coating Layer
Sacrificial metal coatings protect the substrate metal by means of galvanic
action. When the base metal is exposed to the atmosphere as ar esult of
mechanical damage or the like, the exposed portion of the basem etal is
polarizedc athodically to the corrosion potential of the coating layer.A sa
result, little corrosiontakes place on the exposed metal. Agalvanic couple is
formed between the exposed part of the base metal and the surrounding
coating metal. Sacrificial metals are more negativei ne lectrochemical
potential than other metals such as iron or steel. Therefore, the sacrificial
metal coating acts as ac athode. Thist ype of reaction of sacrificial metal
coatings is known as galvanic or cathodic protection. In addition,the defects
are protected by as econd barrier of corrosion products of the coating
layer.F igure 7.4s chematically illustrates the galvanica ctiono fasacrificial
metal coating.
Coatings
413
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485