Coatings for Corrosion Protection
5.2 Properties of Organic Coatings
5.2.1 Electrical and Electrochemical Properties .1 Review of the Types of Reactions
Figure 5-1 shows schematically the processes that can take place on coated metals in electrolytes. The following properties are important:
Fig. 5-1 Over- view of reactions on coated steel.
• permeability of the coating to corrosive materials permeation of molecules (O2, H2O, CO2, etc.), migration of ions (anions and cations);
• mechanical damage to the coating that exposes the metal surface, allow- ing electrochemical corrosion reactions to take place.
Ion migration can be explained by Eq. (2-23). The electrical voltages involved range from a few tenths to several volts and arise from the following causes [8-10]:
Anodic polarization: exiting stray currents, contact with foreign cathodic structures;
Cathodic polarization: entering stray currents, cathodic protection.
A consequence of ion migration is electrolytic blister formation. In the case of anodic blisters the coated surface shows pitting, whereas in the case of cathodic blisters there is no change in the metal surface or there is merely the formation of thin oxide layers with annealing color.
An important consequence of ion migration is the formation of cells where the coated surface acts as a cathode and the exposed metal at the damage acts as an anode (see Section 4.3). The reason for this is that at the metal/coating interface, the cathodic partial reaction of oxygen reduction according to Eq. (2-17) is much less restricted than the anodic partial reaction according to Eq. (2-21). The activity of such cells can be stimulated by cathodic protection.
In the case of free corrosion at the rim of the holidays and in the case of ca- thodic protection on the entire surface exposed (to the soil), oxygen reduction and production of OH~ ions take place according to Eq. (2-17). The local pH is there- fore strongly increased. The OH" ions are in a position to react with the adhesive groups in the coating and thus migrate under the coating. This process is known as cathodic disbonding, or more appropriately, alkaline disbonding. The parameters influencing the various types of reaction and the requirements for corrosion pro- tection are dealt with in detail in the following sections.
5.2.1.2 Coating Resistance and Protection Current Demand
The following terms apply to the specific coating resistance which is related to the surface, S: r* is the value calculated from the specific resistance pD of the coat- ing material using Eq. (5-1):
Table 5-1 Comparison of specific coating resistances.
Coating p* s rG ' D u u ux r°b s rc
material (Q, cm) (mm) (Q m2) (Q m2) (mm) (Q m2) Bitumen [4] >1014 4 4 x l 09 3 X 105 4 to 10 -104 PE[4] 1018 2 2 x l 01 3 1011 2 to 4 -105
EP[4] 1015 0.4 4 x l 09 108 0.4 -104
PUR tar [2] 3 x 1014 2 6 x l 09 109 2.5
a From Ref. 11; for PUR tar from Ref. 12.
b From Table 5-2.
c See Fig. 5-3.
where s is the thickness of the coating, r° is the value obtained in laboratory or field experiments for a defect-free coating in contact with the medium, and ru is the value obtained on structures in the medium in service where there are usually pores and holidays present.
Its determination follows from current and potential measurements:
With large resistances and at high voltages the replacements U = At/ and / = A/
can be made (see Refs. 2-5). At lower voltages and for buried objects (Uon - t/off) = At/ can be replaced with / = A/ [8]. Such a measurement is described in Section 3.4.3 [see Fig. 3-13 and Eq. (3-40)].
The r* values from Eq. (5-1) for the most important materials for pipe coatings are given in Table 5-1. Table 5-2 contains results of long-term field experiments.
For comparison, the values of r^are included in Table 5-1. It can be seen that r°
values are always smaller than r* values, which is apparently due to the absorption of water when the coating is immersed in the medium. A marked reduction in the coating resistance has been observed with increasing temperature for resins [9,13,14]
(see Fig. 5-2 [14]).
Figure 5-3 is a compilation of coating resistances for long-distance pipelines as a function of service life for PE and bitumen [15]. The ru values for objects in service are considerably lower compared to the T* or r° values. This is due to pores or holidays in the coating and to poorly coated fittings and defects in the coating of the girth welds, where the metal is exposed to the environment. By neglecting polarization resistances, the resistance 7?x of a defect, with a diameter d, in a coat- ing of thickness s, and with a medium of specific resistance p, is obtained from the sum of the pore resistances RF
Fig. 5-2 Influence of temperature on the coating resistance r^ when 3-mm- thick PE and 0.4-mm-thick EP coatings are immersed in water.
Fig. 5-3 Coating resistances of long- distance pipelines.
free coating.
Coating
PE (2x), soft adhesive PE (2x), soft adhesive PE molten adhesive PE molten adhesive PE melted
PE melted Bitumen Bitumen Bitumen Bitumen EP tar EP tar PUR tar EP
PE tape, soft adhesive PE tape, soft adhesive PE tape system PE tape system PE tape system PE tape system
PE tape without adhesive PE heat-shrunk sleeve
7 4 2.4 2.2 4 2.2 7 4 10 6 2 2 2.5 0.4 0.9 0.9 3 3 3.5 1.5 0.7 3
Soil Water
Soil Water
Soil Water
Soil Water
Soil Water
Soil Water
Soil Soil Soil Water
Soil Water
Soil Soil Soil Soil
0.2 10
<o.
1 190.2 10
<o.
1 190.2 10 0.4 19 0.2 10 0.2 19 ND 10 0.1 19 0.2 10 0.1 19 ND 10 ND 10
0.4 10 0.4 19 ND 10 0.2 19 ND 10 ND 10 ND 10 ND 10
10"
10"
3 x 10'0 10"
3 x 10'0 3 x 10'0 105 3 x 105 3 x 105 3 x 105 3 x 107 109 107 107 107 3 x 109
109 3~ 104
107 3 x 106
108
108
20 19 20 19 20 19 20 19 20 19 20 19 10 10 20 19 20 19 10 10 10 10
Precursor of DIN 30670 Precursor of DIN 30670 DIN 30670
DIN 30670
Precursor of DIN 30670 Precursor of DIN 30670b DIN 30673'
DIN 30673' DIN 30673' DIN 30673'
Fiberglass reinforced Fiberglass reinforced DIN 3067 1
DIN 30671
Precursor of DIN 30672b Precursor of DIN 30672b DIN 30672
DIN 30672 DIN 30672 DIN 30672 50% overlap DIN 30672
ND = Not determined.
The disbonding is clearly time dependent.
The coating resistance decreases slightly with time.
a Water: pipe length 2 to 4 m, 1.3 m deep; soil: pipe length 12 m, 1 m covering.
Thickness t r: t
(mm) Mediuma ( P a-'1 (a) (SZ m2) (a) Remarks
and the grounding resistance RA - p/(2d) from Eq. (24-17):
z,c-t \ /it* /
If there are several defects, the total resistance Rg of the individual resistances connected in parallel gives from Eq. (5-4):
/CC* .•
If the defect diameters are all the same d - d(, it follows from Eq. (5-5):
where n is the number of defects.
In general, it cannot be assumed that the defect diameters are all the same and larger than the coating thickness (dt > s). For an average diameter
and from Eq. (5-5):
By introducing the defect density N = n/S, the specific total resistance of the defects rg = Rg S becomes:
(a) equal d{ from Eq. (5-6):
From the parallel connection of r° for the intact coating and r for the defects, it follows finally that:
Because r° is usually considerably larger than rg, ru is practically determined only by the defects (see Table 5-1).
A relation can be derived between ru and the necessary protection current den- sity Js from Eq. (5-2) together with the pragmatic protection Criterion 2 in Table 3-3 [see Eq. (3-31)]:
Like Criterion 2, this relation has no theoretical foundation and only serves as a comparison and for the design of protection installations. For this reason Js in Eq. (5-11') is sometimes less correctly termed the conventional protection current requirement.
If rg is only the result of defects, there is the question of a connection between rg
and the total area of defects 50. With SJS = N(nd2/4) and Eq. (5-9b), it follows that:
With the protection current density J°s for the uncoated surface S0, the protection current density Js is given by
o
from which, with Eq. (5-12):
Equation (5-11) cannot be explained by Eq. (5-14) because J°s and p are variables related to the soil and furthermore the average defect diameter, d, is not constant.
If, however, it is assumed from Eq. (2-40) that the protection current density corresponds to the cathodic partial current density for the oxygen reduction reac- tion, where oxygen diffusion and polarization current have the same spatial distri- bution, it follows from Eq. (2-47) with rg = A0/7:
This equation also includes, as in Eq. (5-14), variables related to the soil, but no data on the defects. A theoretical basis for Eq. (5-11) is also not possible. Equa-
tion (5-11) can therefore give at most only approximate information on the expected protection current requirement.
Protection current density and coating resistance are important for the current distribution and for the range of the electrochemical protection. The coating resis- tance determines, as does the polarization resistance, the polarization parameter (see Sections 2.2.5 and 24.5). For pipelines the protection current density deter- mines the length of the protection range (see Section 24.4.3).
5.2.1.3 Effectiveness of Cathodes and Cell Formation
Cell formation can easily be detected by measuring potential if coated surfaces with no pores have a more positive potential than uncoated material. Usually this is the case with coated steel in solutions containing oxygen. More negative poten- tials can only arise with galvanized steel surfaces. Figure 5-4 shows examples of measured cell currents [9,10,16].
The cell current is determined by the coating resistance rjj and the size of the coated surface Sc. Neglecting the anode resistance, the cell current from these two quantities is given by [see Eq. (2-43)]:
Dividing by the anode surface 50 and with rg = ru, S = 5C and Eq. (5-12) gives:
Fig. 5-4 Cell currents between a coated specimen (Sc = 300 cm2) and uncoated steel electrode (5a = 1.2 cm2) in NaCl solutions at 25°C. Left: shot-peened steel sheet, 150 /mi EP-tar. Right: hot-dipped galvanized steel sheet, 150 /im EP-tar.
The importance of large r° values becomes obvious from Eq. (5-16) but in making an assessment, the size of the object (Sc) must also be considered (see Ref. 8). The r^
values, especially with thin coatings, can lie in a wide range, from 102 to 107 Q. m2. Thus the prime coating and the total coating thickness have a considerable influence [16]. Equation (5-17) can be applied when assessing pipelines [17]. The ratio ru/r°
has a great effect. The figures in Table 5-1 indicate that in contrast to a bitumen coating, there is certainly no danger of cell formation with PE coating as long as the pipeline is not electrically connected to foreign (cathodic) structures. The assertions in Ref. 18 about sufficient external protection for installations that are to be protected but that are without cathodic protection, are in agreement with these observations.
5.2.1.4 Electrochemical Blistering
Electrochemical blistering is a result of the ion conductivity of the coating mate- rial. It is only expected with thin coatings with sufficiently low r® values which have been immersed in a medium for long periods of service [8-10]. Cathodic blisters are the best known and occur in saline solutions with cathodic protection. What is necessary is the presence of alkali ions and the permeation of H2O and O2 so that OH" ions are formed in the cathodic partial reaction, as in Eq. (2-17) or (2-19) at the metal/coating interface, forming a caustic solution with the migrating alkali ions. H2O diffuses to the place where the reaction is occurring by osmosis and electro-osmosis, and leads to the formation of relatively large blisters. Anodic blisters can also be observed where there is contact with foreign cathodic objects. The migration of anions is necessary to form soluble corrosion products with the cations of the underlying metal. The cations are formed with anodic polarization according to Eq. (2-21). Osmotic and electro-osmotic processes act against H2O migration, so that anodic blisters are considerably smaller than cathodic blisters. Pitting corrosion always occurs at anodic blisters.
Blisters are apparently statistically distributed and their formation is connected with paths of increased ion conductivity in the coating material. Whether this is a question of microporosity is a matter of definition. Since the skin of the blister remains impervious to water in such micropores, the term "pore" for conducting regions is avoided in this handbook, especially as the properties of a pronounced pore are the same as those of a damaged area (holiday).
If the metal surface is insufficiently cleaned before coating and contains local salt residues, osmotic blistering is to be reckoned with because it enhances electro- chemical blistering and determines the points where it occurs. This is also appli- cable to the action of ions in the prime coating. Coating systems with an alkali silicate primer coat are particularly likely to form cathodic blisters [16]. Blisters often form in the vicinity of mechanical damage (see Fig. 5-5 [19]). With strong cathodic polarization in limiting cases, complete disbonding can occur.
Fig. 5-5 Cathodic blisters in the vicinity of a scratched cross; steel pipe with 70 ]Jm Zn-epoxy resin +300 ]nm EP-tar, seawater; f/Cu.CuS04 = -1.1 to -1.2 V, 220 days, 20°C.
There are numerous publications [9,10,16,19-24] and test specifications [8,25]
on the formation of cathodic blisters. They are particularly relevant to ships, marine structures and the internal protection of storage tanks. Blister attack in- creases with rising cathodic polarization. Figures 5-6 and 5-7 show the potential dependence of blister density and the NaOH concentration of blister fluid, where it is assumed that c(Na+) and c(NaOH) are equal due to the low value of c(Cl~) [23].
The blister population-potential curves can intersect one another. Thus short- term experiments at very negative potentials, in the region of cathodic overprotec- tion, give no information on the behavior at potentials in the normal protection range. The susceptibility generally increases strongly with overprotection (f/H<-0.83V).
Compared with cathodic blisters, which can be recognized by their alkali con- tent, anodic blisters can be easily overlooked. Intact blisters can be recognized by the slightly lower pH value of the hydrolyzed corrosion product. The pitted surface at a damaged blister cannot be distinguished from that formed at pores.
In general the population of cathodic blisters increases with cathodic polariza- tion and the population of anodic blisters with anodic polarization. Both types of
Fig. 5-6 Relation between the density of cathodic blisters and potential; shot-peened steel sheet without primer (•) and with about 40 /mi primer coat (Zn ethyl silicate + poly- vinyl butyral) (•); top coat: 500 /am EP-tar;
0.5 M NaCl, 770 days at 25°C.
Fig. 5-7 Effect of potential on the composition of the blister liquid, shot- peened pipe with 300 to 500 jUm EP-tar, artificial seawater, 1300 days at 25°C.
blisters can occur near one another in free corrosion. Since ion conductivity is essential for the formation of these blisters, there is a correlation between r°u and susceptibility to blistering. In a large number of investigations with EP-tar coat- ings on steel coupons in NaCl solutions, test pieces with an average value of rJJ greater than 106 Q m2 remained free of blisters. Test pieces with r°u < 103 Q m2 were attacked [16].
Blisters or disbonding can occur with long-term cathodic polarization in solu- tions without alkali ions and/or in coatings with sufficiently high rJ values where the reaction of the blister liquid is neutral. In the case of adhesion loss on a larger area, as in the right-hand picture in Fig. 5-8, the presence of humidity (water) could only be detected by the slight rusting. In these cases electrolytic processes are not involved but rather electro-osmotic transport of water with cathodic polarization [9,10] based on Cohen's rule [26]. According to this rule, the macrogel coating material is negatively charged compared with water. The contrary process, anodic dehydration, is responsible for the small extent of anodic blisters. It can also be recognized by the tendency of areas in the neighborhood of anodes to dry out (see Section 7.5.1). No blisters are formed in anodic polarization in solutions without such anions, which can form easily soluble corrosion products with cations of the bases metal, or at high rj values. With thin coatings after long-term anodic treat-
ment of steel, small dark specks can be seen which consist of Fe3O4. Pitting corro- sion is not observed.
5.2.7.5 Cathodic Disbanding
The production of OH~ ions according to Eq. (2-17) or (2-19) in pores or dam- aged areas is responsible for cathodic disbonding [9,10], where the necessary high concentration of OH" ions is only possible if counter-ions are present. These in- clude alkali ions, NHJ and Ba2+. Disbonding due to the presence of Ca2+ ions is extremely slight [27].
Disbonding in free corrosion depends on the formation of aeration cells which produce OH~ ions in the cathodic region according to Eq. (2-17). Correspondingly, exclusion of oxygen or inhibiting corrosion can prevent disbonding [19]. No disbonding takes place in a K2CrO4 solution because there is no corrosion [27].
Addition of acids also suppresses disbonding by neutralizing OH" ions, but encour- ages acid attack at holidays. On the other hand, alkalis strongly promote disbonding [19] (see Fig. 5-9 [10]) but favor corrosion resistance at holidays.
With anodic polarization, the anodic partial reaction predominates at defects so that OH~ ions formed according to Eq. (2-17) are combined in the corrosion
Fig. 5-8 Total adhesion loss of a 500-/im-thick coating of EP (liquid lacquer), 0.2 M NaCl, galvanostatic Jv = -1.5 ^A m~2, 5 years at 25°C. Left: coating with a pin pore; loss of adhesion due to cathodic disbonding. Right: pore-free coating;
loss of adhesion due to electro-osmotic transport of H2O. In both cases the loose coating was removed at the end of the experiment.