Universidad Nacional de Chimborazo
NOVASINERGIA 2018, Vol. 1, No. 2, junio-noviembre (5-19)
ISSN: 2631-2654
https://doi.org/10.37135/unach.ns.001.02.01
Research Article
http://novasinergia.unach.edu.ec
Optimum selection of zinc-coated cable trunking systems for
electrical installations based on atmospheric corrosion prediction
Selección óptima basada en la predicción de corrosión atmosférica de sistemas
de conducción de cables eléctricos con recubrimiento de zinc para
instalaciones eléctricas
Ernesto Chenoll-Mora
1
*, Vicente-Agustín Cloquell-Ballester
2.
María-Cristina Santamarina-
Siurana
2
1
Engineering Projects Department. Universitat Politècnica de València. València, Spain, 46003
2
ACUMA Research Center, Universitat Politècnica de València. València, Spain, 46003; cloquell@upv.es;
csantama@agf.upv.es
* Correspondence: ernesto.chenoll@icloud.com
Recibido 19 junio 2018; Aceptado 18 agosto 2018; Publicado 10 diciembre 2018
Abstract:
The study presents a methodology for the optimum selection of the most
suitable zinc-based coatings in metallic trunking systems to fulfill the
requirements related to atmospheric corrosion resistance. The current
methodologies are based on heuristic procedures that do not consider the
influence of the in situ atmospheric conditions, which are the main cause of
most of the corrosion problems. The effect of corrosion over time is generally
estimated using a logarithmic function, which depends on corrosion during the
first year of exposure, as well as on environmental parameters (e.g.
temperature, humidity, pollutants, among others). Different mathematical
models for the prediction of corrosion during the first year of exposure were
analyzed. Ten of these models were selected and compared with actual tests
determining the model that best fitted the actual values. From this first-year
corrosion value, the long-term corrosion function was calculated for each
relevant commercial coating. Finally, a case study was analyzed by means of
the proposed methodology. The results show the importance of the corrosion
function and its influence in the selection of the coating to minimize costs.
Keywords:
Atmospheric corrosion, coatings, trunking, design, project
Resumen
Se presenta una metodología para la selección óptima del recubrimiento a base
de zinc más adecuado en sistemas metálicos de canalización, para cumplir con
aquellos requisitos de un proyecto industrial relacionados con la resistencia a
la corrosión atmosférica. Las actuales metodologías están basadas en
procedimientos de cálculo heurísticos, que no consideran la influencia de las
condiciones atmosféricas in situ y que son la principal causa de la mayoría de
los problemas de corrosión. El efecto de la corrosión a lo largo del tiempo,
generalmente se estima utilizando una función logarítmica, que depende de la
corrosión durante el primer año de exposición, así como de los parámetros
ambientales (ejemplo: temperatura, humedad, contaminantes, entre otros). Se
han analizado diferentes modelos matemáticos para la predicción de la
corrosión durante el primer año de exposición. Diez de estos modelos han sido
seleccionados y comparados con ensayos reales, para finalmente seleccionar
el modelo que mejor se ajusta a dichos valores reales. A partir de este valor de
corrosión del primer año, se calcula la función de corrosión a largo plazo para
cada revestimiento comercial relevante. Finalmente, se analiza un caso de
estudio mediante la metodología propuesta. Los resultados muestran
claramente la importancia de la función de corrosión y su influencia en la
selección del recubrimiento que minimiza el costo.
Palabras
clave:
Corrosión atmosférica, recubrimientos, canalización, diseño, proyecto
http://novasinergia.unach.edu.ec 6
1 Introduction
1.1 Background
The lack of analytical methods in the field of
industrial electrical trunking systems
1
, for
determining corrosion effects on metal coatings,
makes optimal coating selection difficult, since
current methods don´t use any scientific
methodology that considers the different
environmental parameters that take part in the
corrosion of the coating. In this regard, the
coating selected in a heuristic way, usually does
not meet the requirements regarding corrosion
resistance; thus, the expected life of the trunking
system could be drastically reduced or, on the
contrary, could be unnecessarily overqualified.
In order to minimize these problems, it is
necessary to provide a methodology for
calculating atmospheric corrosion, considering
all parameters that take part in this process
including meteorological factors (e.g. relative
humidity, number of rainy days, temperature,
among others) and pollutants (mainly, chlorine
and sulphur ions).
1.2 Manufacturers’
recommendations
Overall, the technical literature from
manufacturers generally includes a
categorization of the type of environments and
the recommended coating for each, as it is shown
in Table 1 (Chenoll Mora, 2005). However, no
manufacturer provides any scientific method to
accurately determine the atmospheric corrosion
of the metal, considering the meteorological and
pollutant parameters of the location.
1.3 The quantification of
atmospheric corrosion:
Current methodologies
1.3.1 Logarithmic general expression
The current methods of quantitative calculation
for atmospheric corrosion are generally based on
two steps:
1
‘
Cable tray systems’ according to IEC 61537 (International
Electrotechnical Commission, 2016), ‘Cable trunking and
cable ducting systems’ according to EN 50085-1 (CEN,
European Committee for Standardization, 2005) and
- Calculation of corrosion after one year (first
year of exposure)
- Calculation of the corrosion for any period of
time (beyond one year of exposure)
As shown in previous studies (CEN, European
Committee for Standardization, 2012; Feliu
Batlle, Morcillo, & Feliu, 1993a; González
Fernández & Consejo Superior de
Investigaciones Científicas (CSIC), 1984;
Pourbaix, 1982b), the corrosion in most of the
cases, is estimated by means of bi-logarithmic
expressions of the type:
(1)
where,
-
is the accumulated corrosion at year .
- is the corrosion at first year of exposure.
- is a constant, which depends on each metal
and the particular atmospheric condition
(Morcillo, 1998); generally, < 1.
- is the time in years.
The non-linearity of corrosion function,
, is
a key element in understanding the corrosion
process over time. In fact, this non-linear model
positively modifies the estimated lifetime of the
installation and consequently, its economic
impact.
Likewise, it is well known that the corrosion
process is, in most cases, stabilized for
having a linear behaviour. Therefore, the
corrosion function can be obtained as follows
(CEN, European Committee for Standardization,
2012; Morcillo, 1998; Panchenko & Marshakov,
2016):
(2)
1.3.2 Corrosion during the first year of
exposure (A)
Following a literature review on different
methodologies, ten studies were selected in order
to determine the best fitting model to actual
corrosion values. Table 2 shows the different
variables and parameters considered on those
techniques.
‘Conduits systems’ according to EN 61386-1 (CEN,
European Committee for Standardization, 2008a).
http://novasinergia.unach.edu.ec 7
Table 1: Summary of coatings recommended by the main cable tray manufacturers (Chenoll Mora, 2005).
Corrosivity
category
ISO 9223
Environments
defined by
manufacturers
Type of coatings
Electro-
plated
(ISO
2081)
Bi-
chromate
electro-
plated
(ISO
2081)
Pre-
Galv.
(EN
10346)
Hot-Dip
Galv.
(ISO
1461)
Stainless
steel
AISI
304
Stainless
steel
AISI 316
Epoxy-
polyest
er
Rilsan
®
(PA)
Levasint
®
(PE)
Alumi
nium
PVC
Galv.
+
epoxy
C1: Very
low
In-door
(normal
environment)
S
O
S
O
Chemical
industry /
Aseptic
I
S
I
S
I
Any C1
environment
S
O
S
O
C2: Low
Out-door
(normal
environment)
I
P
(1)
S
O
P
O
P
Food industry
I
S
O
I
S
Levasint
®
S
I
Abrasive
environment
P
S
O
S
S
Levasint
®
O
P
S
Any C2
environment
I
P
(1)
S
O
P
O
P
O
C3:
Medium
Alkaline
environment
I
P
S
P
S
P
Hydrocarbons
I
P
S
NIA
Organic acids
I
P
S
NIA
Out-door
moderated
severity
I
P
S
NIA
Any C3
environment
NIA
I
S
O
NIA
O
C4: High
Acid
environment
I
P
S
P
S
I
S
P
Seashore
I
P
I
S
NIA
Mineral acids
I
P
S
NIA
Caustic soda
I
P
S
NIA
Indoor
aggressive
environment
I
P
S
NIA
Any C4
environment
NIA
I
S
O
NIA
O
C5-I: Very
high
(industrial)
Halogen
environment
I
S
P
S
Chlorine
I
P
S
NIA
Industrial
environment
(humid -
sulphurous)
NIA
I
P
S
NIA
Levasint
®
S
NIA
S
Any C5-I
environment
NIA
I
S
NIA
S
C5-M: Very
high
(marine)
Marine
environment,
aggressive,
sulphurous
I
P
S
I
S
P
Any C5-M
environment
NIA
I
S
NIA
S
Note: (S) Suitable; (I) Inadequate; (O) Overqualified; (P) Possible
(2)
; NIA: No Information Available
(3)
(1) The electroplated bi-chromate and the Pre-Galvanized finishes can be used in normal outdoor environments, understood as such,
by dry environments or with very low humidity levels. If it is a clean environment (low pollution), but with the possibility of
reaching high relative humidity, this type of environment would change to a higher classification range, as C3 or C4.
(2) It is possible to use the coating, although it is not the optimal option. In these cases, further important parameters of the installation
should be studied in more depth: humidity, temperature, pollution, temperature gradients, atmospheric conditions, among others,
to ensure that the coating can be used.
(3) The manufacturer who defines this type of environment, does not manufacture this type of finish, so no information is available
in that respect.
(4) Sources: Thorsman, Permisa, Industrias Eléctricas Pinazo, Porime, Latina Canale SRL, NLC Sistema Metallici, Aemsa, Apiem
http://novasinergia.unach.edu.ec 8
Table 2: Variables and parameters used in the methods to estimate annual corrosion (A).
Variable /
Parameter
Description / Value
Units
A
x
Corrosion at first year of exposure calculated with method x
Microns (µm)
RH
Average annual relative humidity
%
T
Average annual temperature
ºC
L
Number of rainy days per year
Days
W
Wetness time estimated, as the number of hours in one year during which RH
80% and T > 0°C simultaneously (ISO, International Organization for
Standardization, 2012)
Hours
M
Corrosion module for 1000 h of wetness of the metal surface in a pure atmosphere
(free of contaminants); for the case of zinc it corresponds to 0.4 m
µm
t
w
Wetness time
Hours/1000
f
t
Coefficient of corrosion inhibition with the annual wetness time (t)
Constant
Influence of SO
2
contamination
Constant
ß
Influence of Cl
-
contamination
Constant
f
c
Stimulating coefficient of corrosion due to contaminants in the air
Constant
Cl
-
:
Average annual concentration of chlorides
mg·(m
-2
·d
-1
)
S
Average annual concentration of sulphur dioxide (SO
2
)
mg·(m
-2
·d
-1
)
S*
Average annual concentration of SO
2
+ Cl
-
mg·(m
-2
·d
-1
)
P
d
Annual average SO
2
deposition
mg·(m
-2
·d
-1
)
f
Zn
0.038·(T – 10) when T <= 10 °C; otherwise, -0.071·(T – 10)
ºC
S
d
Annual average Cl
-
deposition
mg·(m
-2
·d
-1
)
D
Day
-
Method 1: Applicable in atmospheres exempt
from contamination (Chico, De La
Fuente, Vega, & Morcillo, 2010;
Feliu & Morcillo, 1980. 2013;
Morcillo & Feliu, 1987).
A
1
= -0.00603·RH+0.0038·T+0.0093·L+0.597 (3)
Method 2: Applicable in atmospheres exempt
from contamination. This method is
based on the same study from which
Method 1 comes from.
A
2
= -0.000198·W+0.015·T+0.015·L+0.215 (4)
Method 3: Applicable in atmospheres exempt
from contamination (Costa, Mercer,
Institute of Materials of London,
European Federation of corrosion, &
Sociedad Española de Química
Industrial, 1993).
A
3
= 0.12·L – 0.35 (5)
Method 4: Applicable in any type of atmosphere
(Morcillo & Feliu, 1993)
A
4
= M·t
w
·f
t
·f
c
(6)
Where fc is calculated through the following
expression:
(7)
Coefficient ft and parameters and , can be
obtained by the following graphs in Figures 1-3
Figure 1: Variation of f
t
, versus wetness time. Source:
own illustration based on reference (Morcillo & Feliu,
1993).
Figure 2: Variation of
versus mean values of SO
2
.
Source: own illustration based on reference (Morcillo
& Feliu, 1993).
http://novasinergia.unach.edu.ec 9
Figure 3: Variation of
versus mean values of
chlorides. Source: own illustration based on reference
(Morcillo & Feliu, 1993).
Method 5: Applicable in contaminated
atmospheres (Morcillo, 1998;
Morcillo & Feliu, 1993)
A
5
= 0.713+0.0511·Cl
-
(8)
Method 6: Applicable in any type of atmosphere
(Almeida, Rosales, Uruchurtu,
Marroco, & Morcillo, 1999)
A
6
= 2.52·W + 0.02·Cl
-
– 0.05 (9)
Method 7: Applicable in contaminated
atmospheres. This method is part of
the same study as that referenced in
Method 10:
A
7
= 0.785+0.0226·S+0.0501·Cl
-
(10)
Method 8: Applicable in any type of atmosphere
(ISO, International Organization for
Standardization, 2012)
A
8
= 0.0219·P
d
0.44
·e
0.046·RH + fzn
+ 0.0175·S
d
0.57
·e
0.008·RH+0.085·T
(11)
Method 9: Applicable in any type of atmosphere
(Haagenrud, Henriksen, & Gram,
1985)
A
9
= 12.26·W + 0.03·S – 3.05 (12)
Method 10: Applicable in contaminated
atmospheres (Benarie & Lipfert,
1986; Feliu Batlle et al., 1993a;
Feliu Batlle, Morcillo, & Feliu,
1993b; Morcillo, 1998)
A
10
= 0.671 + 0.0741·S* (13)
1.3.3 Estimation of the parameter n
Several examples have been used to determine
the parameter n (equations 1 & 2), of which the
following were selected:
It is commonly accepted (CEN, European
Committee for Standardization, 2012; Chico et
al., 2010; Hernández, Miranda, & Domínguez,
2002) that for the case of zinc, n-parameter is
usually in the range of 0.8 to 1, although this
range depends on the type of environment of the
installation.
For his part, M. Pourbaix (Pourbaix, 1982a)
facilitates reference values, which are showed in
Table 3.
Table 3: Possible values of n-parameter for different
types of atmospheres (Pourbaix, 1982a).
Rural atmosphere
Urban-
Industrial
atmosphere
Marine
atmosphere
0.65
0.9
0.9
M. Morcillo (Morcillo, 1998) makes the analysis
for exposures over 10 years (Table 4), based on
actual field trials within the ISO CORRAG
program (Dean & Reiser, 2002; Knotkova,
Boschek, & Kreislova, 1995; Knotkova, Dean, &
Kreislova, 2010; Panchenko, Marshakov, Igonin,
Kovtanyuk, & Nikolaeva, 2014).
Table 4: n ranges obtained in long-term exposures (10-
20 years) (Morcillo, 1998).
Rural-Urban
atmosphere
away from the
sea
Industrial
atmospher
es away
from the
sea
Marin
e
atmos
phere
0.8 – 1
0.9 – 1
0.7 –
0.9
The standard EN ISO 9224 (CEN, European
Committee for Standardization, 2012), gives two
values for n: B1 and B2 (Table 5). For general
applications, n will take the value of B1. In those
cases, where it is important to estimate a more
conservative corrosion attack limit after long
exposures, the value of B should be increased to
consider the uncertainties of the values of B1.
Value B2 includes these uncertainties. Therefore,
the use of B1 or B2 as the parameter n, will
clearly depend on the degree of accuracy that is
intended for the calculation
Table 5: n-parameter values for predicting and
estimating zinc corrosion attack according to EN ISO
9224 (CEN, European Committee for Standardization,
2012).
B1
B2
0.813
0.873
http://novasinergia.unach.edu.ec 10
Regarding the previous standard, it is also
advisable to use a value of 1 for n, for in the cases
of installations in environments with a high
content of sulphur dioxide, it is assumed that the
corrosion of zinc is almost linear.
From this analysis, the designer must choose the
most appropriate value of n, considering these
general recommendations:
- As a general value, the one established by EN
ISO 9224 (CEN, European Committee for
Standardization, 2012), can be taken.
- For t > 20 years, values between 0.9 and 1
should be chosen, because the zinc corrosion
ratio becomes linear from this exposition time
(CEN, European Committee for
Standardization, 2012).
- For environments with very high
concentrations of sulphur dioxide (P3), values
between 0.9 and 1 should be used (CEN,
European Committee for Standardization,
2012).
- For exposures in rural environments with very
low pollution rates and for exposures around 10
years, lower values should be used for n,
according to the previous tables, but not below
0.65.
Finally, it is advisable to review the information
provided in the aforementioned ISO CORRAG
program, which is used in many research studies
in the field of corrosion (Dean & Reiser, 2002;
Knotkova et al., 1995. 2010; Panchenko et al.,
2014).
2 Methodology
2.1 Comparative analysis of
annual corrosion, between
current theoretical methods
and actual field tests
This section aimed to verify the adequacy of the
current methods used to determine corrosion
prediction for the first year of exposure, versus
actual corrosion values measured in field tests. In
this way, the parameters corresponding to 15
different test stations, each having distinct
atmospheric natures, were used: 13 from the
Iberian Peninsula, 1 in France and 1 in Finland.
From these parameters, the corrosion for the first
year of exposure for each of the 10 above-
mentioned methods (A
x
) was calculated and
compared with the actual values measured at
such test stations.
The results of the analyses are shown in Tables
6-8, including the following information:
- Meteorological and environmental
parameters (RH, T, L, W, Cl
-
, S).
- Results of calculated corrosion values (A
x
) for
each of the methods specified.
- Actual corrosion values (Morcillo & Feliu,
1993; Panchenko & Marshakov, 2016).
- The difference between theoretical predicted
values and the actual results for each of the
methods. Here, methods with the least
differences are highlighted.
- The average of the differences of each
method and its standard deviation, as
fundamental data, to decide the method that
best fits all the analyzed scenarios.
The meteorological and environmental records of
these stations were extracted from the data of the
actual corrosion value field tests (Morcillo &
Feliu, 1993; Panchenko & Marshakov, 2016).
There are also alternative sources to get this
parameters like national meteorological
institutes, web sites (“Weather and Climate:
Average monthly Rainfall, Sunshine,
Temperatures, Humidity, Wind Speed,” n.d.;
“World climate data - Temperature, Weather and
rainfall,” n.d.), etc.
From the results, the following conclusions were
taken into account for designing the proposed
methodology:
- Method 1, is the procedure that best matches
the actual values of corrosion: lowest average
of differences (0.46 μm) and standard
deviation (0.53 μm). This method is only
applicable to rural atmospheres (classes C1 to
C3 according to ISO 9223 (ISO, International
Organization for Standardization, 2012).
- Method 4, is the one that best fits the actual
test values for contaminated atmospheres
(classes C4 to CX according to ISO 9223
(ISO, International Organization for
Standardization, 2012): lowest average of
differences (1.19 μm) and standard deviation
(1.78 μm). This method could be used also for
rural atmospheres.
- The accuracy of the adjustment against actual
values of the methods for rural environments
is quite good, with not one average deviation
exceeding 0.8 microns, therefore, it could be
said that any of these methods could be used.
- Methods 9 and 10 were discarded, since they
predicted theoretical corrosion values very
distant to the actual results.
http://novasinergia.unach.edu.ec 11
Table 6: Predicted corrosion values for the first year of exposure versus actual test stations values (Part I).
Test station
location
Alicante
(Spain, 30 m
from sea)
Alicante
(Spain, 100
m from sea)
El Escorial
(Madrid-
Spain, 1032
m from sea)
Bilbao
(Spain, 6 m
from sea)
Barcelona
(Spain, 13 m
from sea)
Cabo Negro
(Javea –
Spain, 12 m
from sea)
Zaragoza
(Spain, 320
m from sea)
Avilés
(Spain, 139
m from sea)
Variable
Value
Value
Value
Value
Value
Value
Value
Value
RH (%)
65
65
62
82
70
65
61
78
T (ºC)
18.75
18.75
13.75
13.75
16.25
13.75
13.75
13.75
L (days)
91
91
101
153
100
91
94
193
W (hours)
4300
4300
3900
3000
3200
4300
2100
3700
M (0.4 µm)
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
t
w
(thousand
hours per year)
4.3
4.3
3.9
3
3.2
4.3
2.1
3.7
f
t
0.5
0.5
0.65
0.9
0.95
0.5
1
0.7
1.2
0
0
0.5
0.4
0
0.2
0
ß
4.4
0
0
1.8
1.3
4
0
0
f
c
(1++ß)
6.6
1
1
3.3
2.7
5
1.2
1
Cl
-
(mg Cl- /
m
2
.d)
166
25
0
67
45
118
0
0
S (mg SO
2
/
m
2
.d)
155
21
15
101
86
30
57
48
S* (mg / m
2
.d)
321
46
15
168
131
148
57
48
P
d
(mg / m
2
.d)
155
21
15
101
86
30
57
48
S
d
(mg / m
2
.d)
166
25
0
67
45
118
0
0
f
Zn
-0.621
-0.621
-0.266
-0.266
-0.444
-0.266
-0.266
-0.266
"A"
Calculation
method
Value
Diff.
Value
Diff.
Value
Diff.
Value
Diff.
Value
Diff.
Value
Diff.
Value
Diff.
Value
Diff.
A
1
(µm) - 1:
Rural
-
-
1.16
0.44
1.26
1.34
-
-
-
-
-
-
1.20
-0.10
2.06
-0.56
A
2
(µm) - 2:
Rural
-
-
1.01
0.59
1.16
1.44
-
-
-
-
-
-
1.42
-0.32
2.58
-1.08
A
3
(µm) - 3:
Rural
-
-
0.74
0.86
0.86
1.74
-
-
-
-
-
-
0.78
0.32
1.97
-
0.466
A
4
(µm) - 4:
General
5.68
0.62
0.86
0.74
1.01
1.59
3.56
2.04
3.28
-0.38
4.30
4.90
1.01
0.09
1.04
0.464
A
5
(µm) - 5:
Contaminated
9.20
-2.90
-
-
-
-
4.14
1.46
3.01
-0.11
6.74
2.46
-
-
-
-
A
6
(µm) - 6:
General
14.11
-7.81
11.3
-9.69
9.78
-7.18
8.85
-3.25
8.91
-6.01
13.15
-3.95
5.24
-4.14
9.27
-7.77
A
7
(µm) - 7:
Contaminated
12.60
-6.30
-
-
-
-
6.42
-0.82
4.98
-2.08
7.37
1.83
-
-
-
-
A
8
(µm) - 8:
General
4.82
1.48
1.80
-0.20
0.96
1.64
6.75
-1.15
3.56
-0.66
2.93
6.27
1.64
-0.54
3.33
-1.83
A
9
(µm) - 9:
General
54.3
-
48.02
50.3
-48.7
45.2
-42.6
36.8
-31.2
38.8
-35.9
50.6
-41.4
24.4
-23.3
43.7
-42.3
A
10
(µm) - 10:
Contaminated
24.46
-
18.16
-
-
-
-
13.1
-7.52
10.4
-7.48
11.6
-2.44
-
-
-
-
Average value
(µm) (Method
1 to 8)
9.28
-2.98
2.81
-1.21
2.51
0.09
5.94
-0.34
4.75
-1.85
6.90
2.30
1.88
-0.78
3.37
-1.87
Actual value
(µm) /
Corrosivity
category ISO
6.3
C5
1.6
C3
2.6
C4
5.6
C5
2.9
C4
9.2
CX
1.1
C3
1.5
C3
Note: Values in “Difference(Diff.)” fields in bold letter, represent the lowest of the values calculated for each of the 10 methods.
http://novasinergia.unach.edu.ec 12
Table 7: Predicted corrosion values for the first year of exposure versus actual test stations values (Part II).
Test station
location
Cádiz (Spain,
14 m from sea)
Madrid (Spain,
655 m from
sea)
Málaga (Spain,
11 m from sea)
La Coruña
(Spain, 26 m
from sea)
Cáceres
(Spain, 459 m
from sea)
Helsinki
(Finland, 26 m
from sea)
Ponteau
Martigues
(France, 9 m
from sea)
Variable
Value
Value
Value
Value
Value
Value
Value
RH (%)
73
62
61.2
82.5
75.8
80
69.6
T (ºC)
19
13.75
18
13.1
12.8
5.4
15.5
L (days)
88
101
28
130
92
115
53.2
W (hours)
4100
2100
1334
4595
3482
3264
4000
M (0.4 µm)
0.4
0.4
0.4
0.4
0.4
0.4
0.4
t
w
(thousand
hours per year)
4.1
2.1
1.334
4.595
3.482
3.264
4
f
t
0.6
1
1
0.6
0.85
0.8
0.6
0
0.2
0
0
0
0
0.4
ß
0
0
0
0
0
0
5
f
c
(1++ß)
1
1.2
1
1
1
1
6.4
Cl
-
(mg Cl- /
m
2
.d)
48
0
0
0
0
4
241
S (mg SO
2
/
m
2
.d)
47
70
0
0
0
18.9
87
S* (mg / m
2
.d)
95
70
0
0
0
22.9
328
P
d
(mg / m
2
.d)
47
70
0
0
0
18.9
87
S
d
(mg / m
2
.d)
48
0
0
0
0
4
241
f
Zn
-0.639
-0.266
-0.568
-0.220
-0.199
-0.175
-0.391
"A" Calculation
method
Value
Diff.
Value
Diff.
Value
Diff.
Value
Diff.
Value
Diff.
Value
Diff.
Value
Diff.
A
1
(µm) - 1:
Rural
1.09
0.91
1.26
0.14
0.57
0.04
1.41
0.64
1.08
0.22
1.25
0.27
-
-
A
2
(µm) - 2:
Rural
1.01
0.99
1.52
-0.12
0.64
-0.03
1.45
0.60
1.10
0.20
1.37
0.15
-
-
A
3
(µm) - 3:
Rural
0.71
1.29
0.86
0.54
-0.01
0.62
1.21
0.84
0.75
0.55
1.03
0.49
-
-
A
4
(µm) - 4:
General
0.98
1.02
1.01
0.39
0.53
0.08
1.10
0.95
1.18
0.12
1.04
0.48
6.14
-3.94
A
5
(µm) - 5:
Contaminated
-
-
-
-
-
-
-
-
-
-
-
-
13.03
-10.83
A
6
(µm) - 6:
General
11.24
-9.24
5.24
-3.84
3.31
-2.70
11.53
-9.48
8.72
-7.42
8.26
-6.74
14.85
-12.65
A
7
(µm) - 7:
Contaminated
-
-
-
-
-
-
-
-
-
-
-
-
14.83
-12.63
A
8
(µm) - 8:
General
3.24
-1.24
1.88
-0.48
0.00
0.61
0.00
2.05
0.00
1.30
2.77
-1.25
5.20
-3.00
A
9
(µm) - 9:
General
48.63
-46.63
24.80
-23.40
13.30
-12.69
53.28
-51.23
39.64
-38.34
37.53
-36.01
48.60
-46.40
A
10
(µm) - 10:
Contaminated
-
-
-
-
-
-
-
-
-
-
-
-
24.98
-22.78
Average value
(µm) (Method 1
to 8)
3.04
-1.04
1.96
-0.56
0.84
-0.23
2.78
-0.73
2.14
-0.84
2.62
-1.10
8.73
-6.53
Actual value
(µm) /
Corrosivity
category ISO
2
C3
1.4
C3
0.61
C1
2.05
C3
1.3
C3
1.52
C3
2.2
C4
Note: Values in “Difference (Diff.)” fields in bold letter, represent the lowest of the values calculated for each of the 10 methods.
http://novasinergia.unach.edu.ec 13
2.2 Flow-chart
Figure 4 illustrates the methodology proposed for
the optimal selection of a zinc-coated cable
trunking system, against atmospheric corrosion.
Table 8: A differences and standard deviation of
corrosion prediction methods for one year of
exposure.
Method
Average
Diff.
(µm)
Standard
deviation
(µm)
A
1
- 1: Rural
0.46
0.53
A
2
- 2: Rural
0.55
0.71
A
3
- 3: Rural
0.77
0.58
A
4
- 4: General
1.19
1.78
A
5
- 5: Contaminated
3.55
5.34
A
6
- 6: General
6.79
2.83
A
7
- 7: Contaminated
4.73
5.64
A
8
- 8: General
1.58
2.20
A
9
- 9: General
37.87
11.05
A
10
- 10: Contaminated
8.90
8.45
Average value (Method
1 to 8)
1.50
1.87
Note: The lowest average difference and standard
deviation values, for both, rural and contaminated
environments, are highlighted
2.3 Description
The proposed methodology involved nine steps
to calculate the maximum coating life based in
the location and the optimum zinc-coated cable
tray, in order to withstand the prescribed lifetime
of the installation in terms of corrosion
resistance.
(1) Determination of customer requirements
The two most important parameters to consider
in terms of atmospheric corrosion for an
industrial trunking system project are:
• Prescribed lifetime of electrical installation
(in years)
• Maximum cost (economic restriction)
(2) Determination of atmospheric data
(Location)
The environmental parameter wetness time (t
w
or
estimated as W) and the concentration of sulphur
dioxide (SO
2
) and chloride contaminants (Cl
-
)
should be collected.
(3) Initial calculation of annual corrosion
In order to determine the corrosivity category, a
general calculation method (rural or
contaminated areas) is needed. Method 4 was
used through equation (6): A
4
= M·t
w
·f
t
·f
c
This method was selected because it has the
lowest average of differences and the lowest
standard deviation from actual test values (see
tables 6-8).
Figure 4: Methodology flowchart.
(4) Determination of the corrosivity category
The first calculation of corrosion (from step 3),
allowed the initial classification of the
corrosivity category to be obtained from Tables
9 and 10, according to ISO 9223 (ISO,
International Organization for Standardization,
2012).
http://novasinergia.unach.edu.ec 14
Table 9: Corrosion rates for zinc, r
corr
, expressed in
μm·a
-1
for the first year of exposure for the different
corrosivity categories ISO 9223 (ISO, 2012).
Here, the initial classification helped to know if
the area could be classified as rural or
contaminated, for determining the theoretical
method of calculation of annual corrosion needed
in the following step. For this, the criterion of
ISO 9223 (ISO, 2012) reflected in table 10, was
followed:
Table 10: ISO atmosphere corrosivity categories (ISO
9223 2012).
Category
Corrosivity
C1
Very low
C2
Low
C3
Medium
C4
High
C5
Very high
CX
Extreme
(5) Calculation of annual corrosion
Once the corrosivity category of the geographical
area where the installation is located has been
determined (from table 10), there are 2 options:
- If the corrosivity category is C1, C2 or C3
(rural atmospheres), then Method 1 will be
applied, as it is the one that best fits the
predicted values for those categories (Tables
6-8).
- If the corrosivity category is C4, C5 or CX
(contaminated atmospheres), then the value
of corrosion, A
4
, calculated in the previous
step with method 4, shall be accepted as valid.
(6) Determination of the parameter n
The value was determined to be between 0,65 and
1 (see section 1.3.3)
(7) Estimation of maximum coating life
The maximum coating life was estimated using
the general equation (1): C (t) = A·t
n
For this, the value of t was cleared, obtaining the
following expression:
(14)
where variable C corresponds to the average
thickness of each of the standard coatings. By
way of reference, when it comes to cable tray
systems, those expressed in table 11, can be used.
Table 11: Mean thickness of zinc coatings mostly used
in electrical cable tray systems (IEC, International
Electrotechnical Commission, 2006).
Type of coating
Average
common
thickness
(µm)
Electroplated
EN ISO 2081 (CEN, European
Committee for
Standardization, 2008b)
8
Pre-galvanized sheet
EN 10346. ISO 4998 (CEN,
European Committee for
Standardization, 2015; ISO,
International Organization for
Standardization, 2014)
15
Hot dip galvanized sheet
EN ISO 1461 (CEN, European
Committee for
Standardization, 2009)
60
Hot dip galvanized wire
EN ISO 1461 (CEN, European
Committee for
Standardization, 2009)
100
Once t
max
was calculated, if it exceeds 20 years
(see Section 1.3.1), it was recalculated using
equation (2), where the value of t was also
cleared and the following expression was
obtained:
(15)
(8) Representation and analysis of the corrosion
function
After determination of the corrosion function (1)
or (2), in order to extract the relevant
conclusions, the corrosion values C(t) were
calculated for each of the values of t and a
graphical representation [C(t) versus t] was also
made. This allowed the visualization of the
evolution and trend of the corrosion process
values over time and facilitated the designer to
choose the most suitable finish.
It was advisable to perform the same exercise on
the same graph with different values of n, to see
how it could affect its variation in the final
choice, including the most demanding case, that
is, when the parameter n is equal to 1 (purely
linear behaviour).
(9) Application of customer restrictions and
final coating selection
Corrosivity category
r
corr
(µm·a
-1
)
C1
r
corr
≤ 0.1
C2
0.1 < r
corr
≤ 0.7
C3
0.7 < r
corr
≤ 2.1
C4
2.1 < r
corr
≤ 4.2
C5
4.2 < r
corr
≤ 8.4
CX
8.4 < r
corr
≤ 25
http://novasinergia.unach.edu.ec 15
From the analysis derived in the previous step
and the customer requirements established in
step 1, the most suitable coating was selected.
3 Results and discussion
The following case study was chosen to illustrate
the methodology described in our work. The city
of Alicante (Spain), 30 meters from the sea coast,
is an area with high pollution rates, with high
concentrations of sulphurs [1.55 mg·(dm
-2
·d
-1
)]
and chlorides [1.66 mg·(dm
-2
·d
-1
)].
Following the proposed methodology, the
subsequent steps were applied:
(1) Customer requirements
For this case study, the following requirements
were taken:
• Dimensions of the prescribed tray: height
60 mm and width 200 mm
• A 15-year guarantee against corrosion
(2) Determination of atmospheric data
(Location)
Geographical location of the facility: Alicante
(Spain), 30 metres from the sea coast.
In the case of wetness time, it was estimated as W
(equivalent to the time period in which RH> 80%
and T> 0 ºC). As indicated in Table 6, for the case
of Alicante, the value was W = 4300 h.
In the contamination parameters (sulphurs and
chlorides, Table 6), the sulphide pollution data
obtained (S) was 1.55 mg·(dm
-2
·d
-1
) and the
chlorine ions (Cl
-
) was 1.66 mg·(dm
-2
·d
-1
).
(3) Initial calculation of annual corrosion
It was calculated using Method 4, by applying
equation (6). The following variables were
determined in advance:
- M: as seen above, for zinc was 0.4 m.
- t
w
: estimated as W (criterion RH> 80% and T>
0º C). In step (2) it was determined that the
value was 4.3 thousand hours per year.
- f
t
: was obtained from t parameter using the
graph of Figure 1. Applied on the graph a t
value of 4.3, it gave back a value of f
t
= 0.7.
- f
c
: This value was calculated from equation (7).
The values of
and
were extracted from the
graphs in Figures 2-3, by applying the values of
sulphur dioxide and chlorides’ concentrations,
respectively, which were obtained at the same
time from Table 6. Accordingly, these values
were S = 1.55 mg·(dm
-2
·d
-1
) and Cl = 1.66
mg·(dm
-2
·d
-1
). This generated the value of
=
1.2 and the value of
= 4.4. Thus, f
c
= 1 + 1.2 +
4.4 = 6.6.
The annual corrosion was calculated with
Method 4, using equation (6), where, A
4
=
0.4·4.3·0.7·6.6 = 7.95 m.
(4) Determination of corrosivity category
from table 9, the corrosion value calculated in the
previous section (A
4
= 7.95 m) corresponded to
an ISO category of C5 (corrosivity in the range
of 4.2 to 8.4 m).
(5) Calculation of annual corrosion (A)
Since the corrosivity category was C5, the
corrosion calculated using Method 4, was
accepted, i.e., A = A
4
= 7.95 m.
(6) Determination of the parameter n
Considering the installation in a Marine
atmosphere, the parameter n was set to n = 0.90
(see section 1.3.3; Tables 3-5).
(7) Estimation of maximum coating life
The maximum duration of the coating was
calculated, as expressed in the previous section,
through equation (14), where A = 7.95 m, n =
0.9 and C is the nominal thickness of the zinc
layer, which was obtained from the values in
Table 11:
- C
ez
=8 m (electroplated)
- C
pg
=15 m (sheet or band pre-galvanized or
continuously galvanized)
- C
hdg
=60 m (sheet or band hot dip galvanized)
- C
hdgw
=100 m (hot dip galvanized wire)
Consequently, by applying (14), the following
results were obtained:
- t
max (ez)
=1.007 years
- t
max (pg)
=2.025 years
- t
max (hdg)
=9.447 years
- t
max (hdgw)
=16.665 years
Since the values for the duration of corrosion
were ostensibly inferior to 20 years, the
application of equation (15) was not be required.
(8) Representation and analysis of the corrosion
function
The corrosion function that followed the present
case study was:
C = 7.95·t
0.9
http://novasinergia.unach.edu.ec 16
In Table 12, the corrosion function was
developed in two ways, in order to show how n-
parameter can affect the final calculation:
a) When n = 0.9 (Corrosion C)
b) When n = 1 (Linear corrosion), eliminating
the logarithmic component
Table 12: Annual corrosion values for logarithmic and
linear functions (Alicante, Spain).
Year
Corrosion C
(µm)
t
max
Linear
corrosion
(µm)
1
7.95
t
max(ez)
(8 µm)
7.95
2
14.84
t
max
(hdg)
(15
µm)
15.90
3
21.37
-
23.85
4
27.68
-
31.80
5
33.84
-
39.75
6
39.88
-
47.70
7
45.81
-
55.65
8
51.66
-
63.60
9
57.44
t
max
(hdg)
(60
µm)
71.55
10
63.15
-
79.50
11
68.81
-
87.45
12
74.41
-
95.40
13
79.97
-
103.35
14
85.48
-
111.30
15
90.96
-
119.25
16
96.40
t
max
(hdgw)
(100
µm)
127.20
17
101.81
-
135.15
18
107.18
-
143.10
(9) Applications of customer restrictions and
final coating selection
For example, the price of a mesh cable tray (made
in wires), considered within the dimensions
required in the project requirements (60 x 200
mm), resulted in 31 €
(Schneider Electric,
2015). If this price was divided between its
average thickness (100 μm), the cost per μm was
of 0.31 €
.
If the parameter n was not taken into account and
the corrosion was understood as linear, for a 15-
year guarantee on corrosion, the cost per meter of
the tray was 119.25 μm·0.31 €/(μm·m)
-1.
i.e.,
36.96 €·m
-1
. On the contrary, if the logarithmic
factor was taken into account, the cost was 90.96
μm·0.31 €·m
-1.
i.e. 28.19 €·m
-1
.
In total, there was a difference of 8.76 €·m
-1
savings, which implied a really important and
positive economic impact.
Figure 5, shows the annual evolution of
corrosion, with and without logarithmic function.
Figure 5: Comparison of annual evolution of corrosion
with a linear behaviour (Alicante, 30 metres from the
sea coast).
Table 12 as well as figure 5, can be very useful
for the engineering and design functions since
they allow to see in an analytical and visual way,
the evolution of the corrosion and consequently,
make it possible to optimize the type of coating
and its cost.
Since the requirement was to guarantee the
installation against corrosion for a minimum
period of 15 years, from table 12, it can be seen
that such requirement could only be met by a
cable tray with a minimum coating of 90.96 μm
which, going to standard thicknesses values,
corresponded to a 100 μm tray, i.e. a tray made
of wires, also known as a mesh cable tray, whose
calculated corrosion resistance time was t
max
=
16.665 years. Moreover, the nominal thickness of
the same tray, could be reduced to 90.96 μm, or
in other words, a reduction in costs of
approximately 10%. Also, the resulting
environmental impact on the coating process
must be mentioned, since: (1) there is less
material and energy consumption (reduced
thickness) and (2) lower CO
2
emissions into the
atmosphere.
From the results of the previous section, the
optimum selection for the atmospheric
conditions corresponded to a mesh cable tray
with a minimum thickness of 91 μm or another
type of tray with the same prescribed dimensions
that would allow an equivalent finish and
thickness, which in turn can comply with the
economic constraints of the industrial project.
http://novasinergia.unach.edu.ec 17
4 Conclusions and further
research
A concise review was presented for the
calculation methods for short, medium and long-
term zinc atmospheric corrosion predictions. The
results obtained, as well as the analysis of the
study, showed that:
• The calculation for medium and long-term
corrosion accepted by most researchers
today, followed the model established in the
equation: C (t)= A·t
n
(1)
• For the calculation of the annual corrosion, A,
the methods analysed that best fitted the
actual corrosion values were Method 1
(Process 3) for rural atmospheres and Method
4 (Process 6) for contaminated atmospheres
• Selection of parameter n was key in the
calculations and it was highly dependent on
the environmental conditions of the location.
• Type values and general recommendations
are given for the determination of n, based in
the specialized literature and research studies.
• The corrosion function, especially in the first
10 years of exposure, showed a logarithmic
and non-linear behaviour
A selection methodology flowchart (Figure 4),
supported by a case study has been provided,
based on the mathematical algorithms analysed
for the calculation of A and the determination of
n-parameter. This methodology involved
calculating the estimated lifetime for certain
atmospheric conditions and considered the
different standard zinc-coated cable thicknesses
currently marketed.
The fact that the evolution of the corrosion obeys
exponential laws, caused the cost rate of the
installation, obtained by the cost per metre of tray
quotient and its thickness, to also decrease
exponentially as the thickness was increased.
This means that, with small increments in
thickness of the coating, it is possible to
exponentially increase the duration of the
coating.
Consequently, the duration is much greater in
comparison to the extra cost to which this
increase of thickness leads to. With this in mind,
it can be assumed that when using conventional
techniques in many cases, installations with
unnecessary costs could be prescribed. This
aspect is especially relevant in cases where the
parameter n moves away from the unit (rural
areas or pollution-free), because the behaviour of
the corrosion rate is less linear in the first years
of exposure. Likewise, the reduction of the
thickness required for the same duration is
guaranteed, minimizing environmental impacts
(material and energy consumption, emissions,
among others).
This research study focused on zinc, as it is the
most widely used coating in the field of electrical
trunking systems. The extension of this
optimisation methodology to other types of
coatings will be the aim for further research,
based on the methodological procedure provided
in this contribution.
Conflict of Interest
No potential conflict of interest was reported by
the authors.
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