Estimation of models for durability of epoxy. 2 coatings in water ballast tanks. 3 doi:10.1B33/saos.2004.0005. 4. Robert E Melchers^ and Xiaoli Jiang^. 5.
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Estimation of models for durability of epoxy coatings in water ballast tanks
3 doi:10.1B33/saos.2004.0005
4
Robert E Melchers^ and Xiaoli Jiang^
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' CeiHre for Infraslructiire
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^ School of Transportation,
Performance
and Reliability,
Wuhan University
The University
of Newcastle,
of Technology/, Wuhan HuBei,
Australia
PR China
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Key words: Coatings, ballast, tanks, life, durability, maintenance.
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!iWï:i^Oi.:)ijs: i N»W
Melchers 2001) and for prediction o f structural l i f e , i t is
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T h e expected life o f protective coatings is o f interest i n the
desirable to have probabilistic estimates o f coating life. ^j^^^.^ ^y^^^^ ^^^^ ^ ^ j j ^ y ^ ^^^j^ the
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management o f maintenance policies and i n the prediction o f structural hfe f o r ships and coastal facihties. I n practice,
literature and there has been little research attention so far p^j^ ^1^;^ ^yp^ modelhng. A n approach to this is pre-
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there are both i n d u s t r y and professional standard guide-
^^^^^^ herein. T h i s should be useful, i n time, as more data
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lines ( A S / N Z 2 3 1 2 : 2002; D N V 1 9 9 2 ; l A C S 2002; S O L A S
^ ^ ^ ^ ^ ^ ^ ^^^ü^ble
the i n t r o d u c t i o n o f the E n -
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2004; T S C F 2002) for the application o f protective coat-
danced Survey Programme for ballast tanks ( l A C S 2002)
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^ ^ ^ . ^ ^ ^ ^ ^ j l ^ ^ recording o f the results o f periodic
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following
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ings. Some o f these also provide rather general estimates
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o f the expected life o f various types o f coatings but w i t h -
coating inspections
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out very specific details o f t h e h sphere o f application. T h e
'
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A k e y f i r s t s t e p i n d e v e l o p i n g m o d e l s i s t o understand the
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numbers provided i n these guidelines have rather wide
^^^.j^^les involved i n coating l i f e . T h i s means that assessing
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ranges. I n practice, periodic inspection o f in-situ coatings
^^e present state o f knowledge and representing i t i n the
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is the key step i n estimating the remaining hfe and the time
^ ^ ^ ^ appropriate manner, rather than developing ' n e w '
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before remedial action must be taken. T h i s is a pragmatic
j ^ ^ j ^ j ^ ^ ^ ^ j ^ ^ ^ ^ pvzaïczl
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approach but it tends to be reactive rather than proactive
f^,.
ufe include: (a) surface preparation i n c l u d i n g
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as is increasingly desired in modern maintenance m a n -
^ j ^ ^ condition o f the surface p r i o r to coating application;
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agement. F o r modern probabilistically based approaches
(b) eventual d r y film thickness and its variability over
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to the setting o f maintenance pohcies (e.g. Faber and
the surface ('holidays'and'peel-backs'), i n c l u d i n g at plate
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Correspor)ding Auttior: R. E. M e k h e r s Centre for Infrastructure Performance a n d Reliability The University o f Newcastle, Australia Email: r o b . m e l c h e r s @ n e w c a s t l e . e d u . a u
^^^^^^^
matters. Factors o f importance
edges, holes and discontinuities and at welds; (c) severity o f
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the (local) environment, i n c l u d i n g temperature range, r e l -
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ative h u m i d i t y b o t h d u r i n g the coating application process and d u r i n g the coating life, U V radiation, and chemical exposure. O f course, coating f o r m u l a t i o n is a f u r t h e r factor ^ ^ j ^ ^ ^ . ^ continues to be m u c h research and development
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.
, .
m this area. © W o o d h e a d Publishing Ltd
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67 SAOS 2006 Vol. I No. I pp. 1-10
R. E. Melchers and X. Jiang
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Surface preparation is extremely i m p o r t a n t (Flores and M o r c i l l o 1999; S t a f f 1996). Some studies have shown that typically 8 0 - 9 0 % o f ' p r e m a t u r e ' coating failures (however defined) are due to inadequate surface preparation or application error (Johnson 1999). T h e local environment is governed b y structural detail and may i n t u r n have an effeet on such details, f o r example, w i t h horizontal stiffeners typically being more prone to corrosion than vertical surfaces. T h e second step is to relate, preferably i n a quantitative manner, h o w these factors influence coating life. T h e d e f i n i t i o n o f coating hfe is an i m p o r t a n t issue. I t is w e l l k n o w n that deterioration o f coating hfe begins w i t h the occurrence o f very small defects and that these gi'ow i n size and severity w i t h t i m e . M o s t coating assessment p r o cedures i n practice are based on this p r i n c i p l e , using the percentage o f the surface showing coating breakdown as a measure o f deterioration. Cracking due to local stresses i n coatings has been observed i n practice f o r m o d e r n coatings (Eliasson 2003; Perera 1995). E v i d e n t l y , any research tool m u s t relate to the reality o f the various forms o f coating
may then be based on subjective interpolation and extrapolation, sometimes w i t h the aid o f knowledge-based systems ( E m i et al 1994). F i e l d observations have tended to show that percentage coating breakdown as a f u n c t i o n o f t i m e of exposure that can be modelled by a normal d i s t r i b u t i o n , and this has been used by several investigators to p r o pose mathematical models f o r coating breakdown (Pirogov et al 1993; Sakhnenko 1997). Similarly, Yamamoto and Ikegami (1998) proposed that coating life could be taken as a normal random variable. However, they d i d not define what they meant by coating life, and i t appears to have been assumed as a discrete quantity rather than one o f gradual degradation. M o r e recently, Adamson (1999) has used industrial field experience to attempt to correlate field data and o p i n i o n for coating degradation w i t h coating characteristics. Degradat i o n was represented i n terms o f A S T M rust grades and this was f o u n d to be a bilinear f u n c t i o n of length and severi t y o f exposure.
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b r e a k d o w n observed i n practice.
C o a t i n g durability in w a t e r ballast t a n k s
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Coating c o n d i t i o n evaluation systems t e n d to fafl i n t o t w o categories: (i) qualitative and ( i i ) quantitative. T h e
Because the d u r a b i l i t y o f a protective coating w i h depend on the coating system, its application, and on the character-
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f o r m e r are more c o m m o n i n practice. Probably, the most popular system f o r ships is the l A C S (International Assoelation o f Classification Societies) system that simply has 'good', ' f a i r ' , and 'poor', based on the percentage o f corroded surface area. Similar qualitative guides are used i n other application jurisdictions, i n c l u d i n g those governed by N A C E , I S O , and N a t i o n a l Standards Associations.
istics of the system i t is supposed to protect, i t is d i f f i c u l t to obtain sensible d u r a b i l i t y estimates unless the scope is very carefully stipulated. I n the present w o r k , this was achieved by c o n f i n i n g attention to a particular (i.e. c o m m o n l y e m ployed) protective coating system as used f o r water ballast tanks i n commercial ships. I n this way, the m o d e l l i n g resuits m i g h t bear some reasonable relationship to actual field
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I t is generally accepted that 1-2% coating breakdown as measured by surface area is cause f o r i n s t i t u t i n g m a i n tenance. T h i s , o f course, is not necessarily the same as coating ' f a i l u r e ' . F o r some coating types, a shghtly higher level o f localized breakdown m i g h t be tolerated, f o r example, f o r z i n c - r i c h paints. H o w e v e r , i t is appropriate to
practice and observations. Water baUast tanks were chosen because (i) their localized corrosion is a major factor f o r commercial shipping; ( i i ) they are subject to a restricted number and range o f environmental influences; and (ni) there appears to be l i t tie published data f o r the d u r a b i l i t y o f water ballast tank
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also aflow f o r the environment and its potential effect on the areas o f coating breakdown, w i t h more aggressive environments leading to a lower expected coating lifespan. These concepts are embodied i n national or commercial
coatings ( T S C F 1992, 2002). Water baflast tanks i n ships are used to maintain adequate d r a f t and t r i m under particular operating conditions. T h e y are flooded periodicaUy w i t h seawater. T h e y are o f t e n
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standards and these could be used f o r p r e d i c t i n g the lifespan o f metaUic and paint coatings systems (e.g. Francis 2001). Coating thickness has an i m p o r t a n t bearing on the permeability ( o f moisture, m a i n l y ) o f the protective coating. A h other factors such as u n i f o r m , thicker coatings tend to have longer effective fives, p r o v i d e d they are applied i n m u l t i p l e layers and do not lead to coating cracking (Friar et al 2001; L a m b o u r n e and Strivens 1999). O f course, f o r the same n o m i n a l thickness, some coating formulations tend to be more durable than others. M e t h o d s f o r quantitative evaluation o f protective coatings are reviewed by M a r t i n et al (1996). These include direct field (e.g. R o f l i 1995) and laboratory testing (e.g. Scully and Hensley 1994), accelerated laboratory testing
empty and are then o f t e n subject to h i g h h u m i d i t y c o n d i tions, usually w i t h h i g h chloride levels. Because o f their locations w i t h i n a ship structure, they may also be subject to elevated temperatures, either f r o m engine rooms or f r o m solar radiation. I n addition, after some t i m e o f ship operation, the b o t t o m o f water baflast tanks is partially or f u l l y covered w i t h wet detritus originating f r o m sources such as water p o l l u t i o n , particulate air p o f l u t i o n (often o r i g i n a t i n g f r o m cargoes), air-borne dust, and partides f r o m protective coating breakdown. As a result o f these harsh operational conditions, water baflast tanks are k n o w n to be particularly prone to corrosion. Factors cont r i b u t i n g to h i g h localized corrosion o f water baflast tanks include the complexity of the structural layout inside tanks, w h i c h typicafly have b o t h vertical and horizontal plate s t i f f -
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and various subjective estimation procedures. Assessment
eners, the latter particularly prone to coflect and retain
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Estimation of models for durability of epoxy coatings in w a t e r ballast tanks
Table I Scenario f o r c o a t i n g d u r a b i l i t y q u e s t i o n n a i r e Ship and route information
Coating and preparation information
Australian commercial bulk carrier vessel on Australia-SE Asia-Japan route Several years old Generally well-maintained Classification society inspections every 5 years Lower water Ballast tank
Surface preparation to Australian standard requirements 45° chambers on all cut plate edges Normal commercial application quality Good quality epoxy coating 200 jum specified minimum thickness (applied in two applications)
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particulate matter and moisture. T h e structural complexity also adds to the d i f f i c u l t y in p r o v i d i n g good surface preparation for application o f protective coatings. A l t h o u g h not ideal, to avoid complications w i t h the issue o f surface preparation, i t was assumed that current ' i n d u s t r y standard' surface preparation was used (irrespective o f pre-
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cisely how this m i g h t be interpreted). Water ballast tanks suffer f r o m practical difficulties w i t h application o f coatings due interior space and access restrictions and (often) the complexity o f structural layouts. T h e y also make high demands on coatings compared w i t h those i n most other parts o f commercial ships, w i t h high
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temperature ranges, high levels of h u m i d i t y , and the inflow o f suspended solids. T h e demands made on surface p r o tective coatings therefore tend to be higher than typically encountered on the exterior o f commercial ships or i n the holds (except for bulk carriers).
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Coating life s u r v e y m e t h o d o l o g y
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Because i t was not considered practical to carry out quantitative analyses o f coating life as a f u n c t i o n o f period o f exposure and o f the many parameters that affect it (many of w h i c h are not easfly quantified i n any case), recourse was
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made to a more subjective assessment. T h i s was done by seeking out key people i n the protective coating industry and i n 'user' organizations and requesting them to p r o vide their best professional opinion to a small number o f questions. T h e questions were posed as a relatively s i m pie questionnaire. I n the event, few such people could be identified o w i n g to the limited number o f manufacturers o f ship coatings and the l i m i t e d number o f people sufficiently expert on the part o f 'user' organizations. O f the 11 so identified and approached, only 8 considered
themselves sufficiently expert i n relation to water ballast tanks to be able to respond and 6 eventually d i d so, 5 w i t h numerical estimates. A l l were provided on condition that the responses remain anonymous. T h e numerical estimates are therefore only labeled A - D i n the sequel. T h e questionnaire ( A p p e n d i x A ) was kept as simple as possible consistent w i t h the i n f o r m a t i o n being sought. I t was divided into five sections. T h e first section p r o vided the details o f the coating type, surface preparation and application, and vessel type, and various assumptions about its r o u t i n g and maintenance (Table 1). T h i s was f o l lowed by questions dealing w i t h first coating breakdown,
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estimates o f the time to various levels o f coating breakd o w n , and the degree o f confidence respondents had about such estimates. Later questions dealt w i t h the effect o f increasing coating thickness and changes i n the operating environment. Before i m p o s i n g the questionnaire on the prospective respondents, i t was trialled on an industry expert. Several modifications were made as a result. Respondents were also given the o p p o r t u n i t y to make observations and to criticise the questionnaire and the questions posed. T h e responses to these last provided very interesting i n formation and are collected together i n the discussion part of this paper.
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RESULTS
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Respondents were asked to state the most likely first signs of coating failure and h o w long after first application this would be expected to occur. T h e results for plate surfaces, plate edges, and coatings along welds are summarized i n Table 2. N o n e o f the respondents provided confidence limits.
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Table 2 F i r s t signs o f c o a t i n g b r e a k d o w n an d e s t i m a t e d t i m e to f i r s t occurrence Edges
Surfaces
Respondent
Welds
Sign(s)
Time (years)
Sign(s)
Time (years)
Sign(s)
Time (years)
A
Paint creep
0.1-5
Paint creep
1-5
1-5
B
Blistering
0.5-1
Rusting
0.75
C D E
Blistering Blistering Blistering
2-3 3 0-1
Cracking Rusting Rust stains
4-5 3 4-5
Localized corrosion Rusting or blistering Blistering Rusting Pin-hole
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