Tanks and Hold Corrosion Prevention Dnv

CLASSIFICATION NOTESNo. 33.1

DET NORSKE VERITASVeritasveien 1, N-1322 Hvik, Norway Tel.: +47 67 57 99 00 Fax: +47 67 57 99 11

CORROSION PREVENTION OF TANKS AND HOLDS

JULY 1999

Det Norske Veritas AS 1999Data processed and typeset by Division Technology and Products, Det Norske Veritas ASPrinted in Norway by Det Norske Veritas AS02-08-20 09:38 - Cn33.1.doc

7.99.2000If any person suffers loss or damage which is proved to have been caused by any negligent act or omission of Det Norske Veritas, then Det Norske Veritasshall pay compensation to such person for his proved direct loss or damage. However, the compensation shall not exceed an amount equal to ten times thefee charged for the service in question, provided that the maximum compensation shall never exceed USD 2 million.

In this provision Det Norske Veritas shall mean the Foundation Det Norske Veritas as well as all its subsidiaries, directors, officers, employees, agents andany other acting on behalf of Det Norske Veritas.

FOREWORDDET NORSKE VERITAS is an autonomous and independent Foundation with the objective of safeguarding life, property andthe environment at sea and ashore.

DET NORSKE VERITAS AS is a fully owned subsidiary Society of the Foundation. It undertakes classification andcertification of ships, mobile offshore units, fixed offshore structures, facilities and systems for shipping and other industries.The Society also carries out research and development associated with these functions.

DET NORSKE VERITAS operates a worldwide network of survey stations and is authorised by more than 120 nationaladministrations to carry out surveys and, in most cases, issue certificates on their behalf.

Classification Notes

Classification Notes are publications that give practical information on classification of ships and other objects. Examples ofdesign solutions, calculation methods, specifications of test procedures, as well as acceptable repair methods for somecomponents are given as interpretations of the more general rule requirements.

A list of Classification Notes is found in the latest edition of the Introduction booklets to the Rules for Classification ofShips, the Rules for Classification of Mobile Offshore Units and the Rules for Classification of High Speed and LightCraft. In Rules for Classification of Fixed Offshore Installations, only those Classification Notes that are relevant for thistype of structure, have been listed.

The list of Classification Notes is also included in the current Classification Services Publications issued by the Society,which is available on request. All publications may be ordered from the Societys Web site http://exchange.dnv.com.

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CONTENTS

1. Introduction .......................................................................................................................................................................41.1 General ................................................................................................................................................................................41.2 Limitation ............................................................................................................................................................................41.3 Definitions ...........................................................................................................................................................................42. Coating Systems.................................................................................................................................................................52.1 General ................................................................................................................................................................................52.2 Coating system no. I Target useful life 5 years ................................................................................................................62.3 Coating system no. II Target useful life 10 years .............................................................................................................62.4 Coating system no. III - Target useful life 15 years ............................................................................................................73. Specific Applications of Coating Systems........................................................................................................................73.1 General ................................................................................................................................................................................73.2 Ballast water tanks...............................................................................................................................................................83.3 Cargo oil tanks.....................................................................................................................................................................83.4 Cargo holds..........................................................................................................................................................................84. References ..........................................................................................................................................................................85. Various degree of rusting on painted steel surfaces .......................................................................................................9

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1. Introduction

1.1 GeneralThis Classification Note describes the quality levels in regardto coating systems and their applicability in cargo tanks,holds and spaces. The Classification Note is, in general,referred to in the rules, when applicable.

The philosophy of the rules, applied in this ClassificationNote, is briefly described below:

The rules for main class 1A1 reflect regulations contained inthe International Convention for Safety of Life at Sea(SOLAS) and in the guidelines issued by the InternationalMaritime Organisation (IMO) and the InternationalAssociation of Classification Societies (IACS). In 1999 thisincluded:

Ballast water tanks shall in general be coated. Coating specifications for seawater ballast tanks in oil

tankers and bulk carriers, constructed on or after 1 July1998, are subject to approval. The requirements arebased on the June 1996 amendments of SOLAS Reg. II-1/3-2 Corrosion prevention of seawater ballast tanks,IMO Res. A.798(19) and IACS Unified Interpretation(UI) SC122. The rules define the minimum content of aspecification describing the corrosion protection systemfor the ballast tanks.

Cargo holds of bulk carriers shall be coated.

This Classification Note supports and further elaborates therule requirements related to additional class notationsCOAT-1 and COAT-2. The code for the application of thedifferent coating systems is given in the rules for theadditional notations. The Classification Note is intended tobe a supplement to those rules.

1.2 LimitationThis Classification Note covers protection of commoncarbon steel structures against seawater and the marineenvironment, limited to water ballast tanks, oil cargo tanksand holds of bulk carriers. The mentioned tanks and holdsare known to be most susceptible to corrosion and are thus ofprime concern. Product tanks are briefly considered.

The Classification Note does not address protection againstcorrosive cargoes or chemicals.

DNV's approval of a coating specification does not implyany follow-up of surface preparation or coating relatedoperations in the shipyard, unless explicitly agreed.

1.3 DefinitionsCoat: A continuous film of paint from a single application.

Dry film thickness DFT: The thickness of the coatingremaining on the surface when the coating has hardened,usually measured in microns (0,001 mm).

Durability of coating (useful life): Until 3 % breakthrough ofrust on coated surface areas, welds and edge areas,respectively. Plain and large surface areas are to beconsidered separately. See Figure 1. (See ISO Standard4628/3, rust scale Ri 3 - 4, or Rust Grade 5, ASTM D 610-85.)

Hard coating: A coating which chemically converts duringits curing process, normally used for new constructions, ornon-convertible air drying coating which may be used formaintenance purposes. Hard coating can be either inorganicor organic [1].

Nominal dry film thickness NDFT: The dry film thicknessspecified for a coat or a coating system. Further details aregiven under respective coating systems I, II and III.

Maximum dry film thickness: The highest acceptable dry filmthickness without impairing the coating quality.

Shop primer (pre-construction primer): Primer coatingapplied as a thin film (approximately 15 25 microns) toprovide temporary corrosion protection of steel plates andstructures.

Sa 1: Light blast cleaning. Loose mill scale, rust and foreignmatter shall be removed. The appearance shall correspond tothe standard photos designated Sa 1. (This originallySwedish standard SIS 055900-1967 is adopted as ISOstandard 8501-1. It is a pictorial surface preparation standardfor painting steel surfaces. Grades Sa 1 - Sa 3 describe blast-cleaned surfaces.)

Sa 2: Thorough blast cleaning. Almost all mill scale, rust andforeign matter shall be removed. Finally, the surface iscleaned with a vacuum cleaner, clean, dry compressed air ora clean brush. It shall then be greyish in colour andcorrespond in appearance to standard photos designated Sa 2.(See Sa 1.)

Sa 2,5 (Sa 2 1/2): Very thorough blast cleaning. Mill scale,rust and foreign matter shall be removed to the extent that theonly traces remaining are slight stains in the form of spots orstripes. Finally, the surface is cleaned with a vacuum cleaner,clean, dry compressed air or a clean brush. It shall thencorrespond to standard photos designated Sa 2,5. (See Sa 1.It should be noted that Sa 2,5 is closer to Sa 3 than to Sa 2.Sa 2,5 corresponds to NACE grade No. 2 (near white) andSSPC grade SP 10 (near-white).)

Sa 3: Blast cleaning to pure metal. Mill scale, rust andforeign matter shall be removed completely. Finally, thesurface is cleaned with a vacuum cleaner, clean, drycompressed air or a clean brush. It shall then have a uniformmetallic colour and correspond in appearance to standardphotos designated Sa 3. (See Sa 1. Sa 3 corresponds toNACE grade No. 1 (white metal) and SSPC grade SP 5(white).)

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St 2: Thorough scraping and wire brushing - machinebrushing - grinding - etc. The treatment shall remove loosemill scale, rust and foreign matter. Finally, the surface iscleaned with a vacuum cleaner, clean, dry compressed air ora clean brush. It should have a faint metallic sheen. Theappearance shall correspond to standard photos designated St2. (This originally Swedish standard SIS 055900-1967 isadopted as ISO standard 8501-1. It is a pictorial surfacepreparation standard for painting steel surfaces. Grades St 2 -St 3 describe mechanically cleaned surfaces.)

St 3: Very thorough scraping and wire brushing - machinebrushing - grinding - etc. Surface preparation as for St 2, butmuch more thoroughly. After removal of dust, the surfaceshall have a pronounced metallic sheen and correspond tostandard photos designated St 3 (See St 2).

2. Coating Systems

2.1 GeneralThe coating systems I to III may be referred to in connectionwith main class or additional class notations as is currentlyvalid and in future publications of the Rules forClassification of Ships.

The indicated coating systems I to III may be considered asgeneral coating systems recognised as being commonly used.They are not meant to exclude other recognised or newlydeveloped systems.

In 2.2, 2.3 and 2.4 three alternative coating systems, namedcoating system I, II and III, are indicated. The systems arenot only intended to be primarily applicable to water ballasttanks, cargo oil tanks and cargo holds, but also to producttanks (in product tankers), as applicable. Notes about specificapplications for the different areas are given in 3.

A general description of the different coating systems I, IIand III are given below.

Target durability

The target durability is meant to be a rough indication ofdurability of the coating, based on collected experience andinformation. It is to be taken into consideration that thedurability of coatings and the susceptibility to corrosion ofdifferent surfaces within the same tank may vary widely. Thedurability (useful life) of the coating is defined in 1.3. Thespecifications of the coating systems I, II and III representthe minimum quality levels needed to obtain thecorresponding target useful coating life of 5, 10, and 15years.

Coating system

The coating system describes the type of coating that isapplicable. Epoxy based coatings are by far the mostextensively used group of coating types (1999). A greatnumber of commercial products are marketed and sold aspure epoxies, modified epoxies, coal tar epoxies,epoxy mastics, surface tolerant epoxies, and other, moreor less characteristic types.

IMO [1] has introduced the concept hard coating, definedin 1.3. Other recognised hard coating as referred to in thecoating systems I to III could for example be based onpolyurethane, silicate, vinyl, etc.

In general, light colour coatings are recommended. Coatingscontaining tar are dark.

Coats and thickness

Coats and thickness describes the number of coats and thenominal dry film thickness. The application of stripe coatsmay be carried out before or after application of the full coat.The nominal dry film thickness as stated in the coatingsystems I to III is connected to the 80/20 rule or 90/5rule, as described in further detail under the respectivespecifications. To obtain a target minimum DFT, the averagethickness should be specified sufficiently high. E.g., with the80/20 rule, to obtain a minimum DFT of 200 microns, thenominal DFT should be about 250 microns.

Primary surface preparation

Primary surface preparation describes the surfacepreparation of the steel plates before application. Thesuitability and compatibility of the shop primer in thecorrosion prevention system is to be recommended by thecoating manufacturer. The shop primer is to be typeapproved by DNV [9].

Secondary surface preparation

Secondary surface preparation includes preparation ofassemblies or sub-assemblies of steel structures. Typically,this includes surface preparation:

at the block stage at erection or pre-erection of sub-assemblies repair of damage to the applied coating system, like

mechanical or burning damage.

Touch-up priming of areas with non-intact shop-primer is notintended, but may be advantageous, depending on thecoating manufacturer's recommendation.

Clean conditions

Clean conditions describe the degree of cleaning requiredincluding requirements for maximum salt content on thesurface, when relevant.

Thermal and hygrometric conditions

Thermal and hygrometric conditions describe limits forsteel temperature and air humidity under coating applicationand blasting operations. Generally, the requirement of steeltemperature to be above the dew point, takes precedenceover the requirement for air humidity, when relevant.


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2.2 Coating system no. I Target useful life 5 yearsTable 2.2 Coating System No. ITarget durability 5 years.Coating system 1) Epoxy based.

Other recognised hard coating 2).Coats and thickness 1 coat 3).

Total nominal dry film thickness (NDFT) 200 microns4).Primary surfacepreparation

Steel plates shop primed on blast cleaned surface to Sa 2 Sa 2,5 (Sa 2,5 recommended).

Secondary surfacepreparation

Welds and burns mechanically cleaned to minimum St. 3.

Clean conditions Any visible salt contamination, oil, grease, dust, weld smoke or dirt on shop primed or other surface to becoated, shall be removed by cleaning.

Thermal andhygrometric conditions

The thermal and hygrometric conditions related to air humidity and steel temperature shall be within thelimits set by the coating manufacturer.

Comments to Coating system I:1) Light coloured coatings are recommended. Coatings containing tar are dark.2) The selection of a recognised coating may depend on the type of compartment and its function.3) One stripe coat shall be applied on edges, welds and in areas where spraying may not be fully effective.4) Nominal dry film thickness shall follow the 80/20 rule and is for Coating system I, defined as follows:

The average DFT based on measurements shall always be equal to or larger than the NDFT. Up to 20% of the area (measuredpoints) may have a thickness between 100% and 80% of the NDFT, but the measured dry film thickness shall always be larger than80% of the NDFT.The measured DFT shall not exceed the maximum dry film thickness defined by the paint manufacturer.

2.3 Coating system no. II Target useful life 10 yearsTable 2.3 Coating System No. IITarget durability 10 years.Coating system 1) Epoxy based.

Other recognised hard coating 2).Coats and thickness 2 coats 3).

Total nominal dry film thickness (NDFT) 300 microns4) 6).Primary surfacepreparation

Zinc containing, silicate based pre-fabrication primer on surface blast cleaned to minimum Sa 2,5.


Sharp edges to be removed. Damaged shop primer blast cleaned to Sa 2,5, including welds and burns. Intact shop primer can remain.

Mechanical cleaning to St 3 acceptable on block joints and damages5) to the applied coating system.Clean conditions Any visible salt contamination, oil, grease, dust, weld smoke or dirt on shop primed or other surface to be

coated, shall be removed by cleaning. The chloride content on surfaces to be coated shall be within the limit set by the coating manufacturer, if

any.Thermal andhygrometric conditions

Air humidity 85 % and steel temperature 3 C above the dew point during blast cleaning and coatingapplication operations.

Comments to Coating system II:1) Light coloured coatings are recommended. Coatings containing tar are dark. If coal tar epoxy is used, the epoxy to tar ratio shall

normally not be less than 60 to 40. If paint with a less epoxy to tar ratio is used (less epoxy, more tar), the thickness of the coatingmay have to be increased.

2) The selection of a recognised coating may depend on the type of compartment and its function.3) One stripe coat shall be applied prior to each full coat on edges, welds and in areas where spraying may not be fully effective.4) Nominal dry film thickness shall follow the 80/20 rule and is for Coating system II, defined as follows:

The average DFT based on measurements shall always be equal to or larger than the NDFT. Up to 20% of the area (measuredpoints) may have a thickness between 100% and 80% of the NDFT, but the measured dry film thickness shall always be larger than80% of the NDFT.The measured DFT shall not exceed the maximum dry film thickness defined by the paint manufacturer.

5) Only applicable for a reasonable amount of damages. Otherwise the basic surface preparation shall be re-applied.6) The NDFT depends on the type of coating. Refers to epoxy based coatings only.


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2.4 Coating system no. III - Target useful life 15 yearsTable 2.4 Coating System No. IIITarget durability 15 years.Coating system 1) Epoxy based

Other recognised hard coating 2).Coats and thickness 2 3 coats 3).

Total nominal dry film thickness (NDFT) 300-400 microns4).Primary surfacepreparation

Zinc containing, silicate based pre-fabrication primer on surface blast cleaned to minimum Sa 2,5.


Sharp edges to be removed. Damaged shop primer blast cleaned to Sa 2,5, including welds and burns. Intact shop primer shall be

sweep blasted. Mechanical cleaning to St 3 is acceptable on block joints and damages to the appliedcoating system.

Clean conditions Any visible salt contamination, oil, grease, dust, weld smoke or dirt on shop primed or other surface to becoated, shall be removed by cleaning.

The chloride content on surfaces to be coated shall be less than 50 mg/m2 (as NaCl) according to Bresleconductimetric method, or an equally recognised method.

Thermal andhygrometric conditions

Air humidity 85 % and steel temperature 3 C above the dew point during blast cleaning and coatingapplication operations.

Comments to Coating system III1) Light coloured coatings are recommended. Coatings containing tar are dark. If coal tar epoxy is used the epoxy to tar

ratio shall not be less than 60 to 40.2) The selection of a recognised coating may depend on the type of compartment and its function.3) One stripe coat shall be applied prior to each full coat on edges, welds and in areas where spraying may not be fully

effective.4) Nominal dry film thickness shall follow the 90/5 rule and is for Coating system III, defined as follows:

The average DFT based on measurements shall always be equal to or larger than the NDFT. Up to 5% of the area(measured points) may have a thickness between 100% and 90% of the NDFT, but the measured dry film thickness shallalways be larger than 90% of the NDFT.The measured DFT shall not exceed the maximum dry film thickness defined by the paint manufacturer.

5) Only applicable for a reasonable amount of damages. Otherwise the basic surface preparation shall be re-applied.

3. Specific Applications of Coating Systems

3.1 GeneralSpecific applications of Coating systems I, II and III aredescribed in 3.2 to 3.4.

Environmental aspects are currently becoming increasinglyimportant for the development and the selection of coatingmaterials. E.g., the use of coal tars is prohibited in somecountries due to their toxicity; and a successive exclusion ofcoal tar epoxies in shipbuilding, in other countries, may beforeseen.

Another trend due to environmental considerations is toreduce the solvent content in coating materials as far as ispossible. This is to reduce the release of volatile organiccompounds into the atmosphere. This trend results in the useof solvent free, high solids or low solvent containing coatingmaterials. These materials may have a potential for the build-up of high dry film thickness, including local over-thickness.

Environmental restrictions are also put on the use of abrasiveblast cleaning. This implies an emerging market for highpressure water jetting or hydro blasting as an alternativesurface preparation to blast cleaning, although mainly formaintenance coating of ships in operation. An advantagewith hydro jetting is, provided clean fresh water is used, thatthe salt content on the surfaces to be coated can be reducedto a very low level. Since salts are recognised as beingdetrimental to the coating quality, promoting blistering andunder-rusting, the lack of anchor pattern by hydro jettingcompared with blast cleaning may to some degree becompensated for by the salts being removed.

The epoxy content in coatings, especially in coal tar epoxies,may be varying. The quality and thus the durability of theapplied coating film may to a large degree be dependent onthe percentage of epoxy resin materials in the coating. Thecontent of epoxy resin (A + B components altogether) in thedry film should not be less than about 40 %. The figure of 40% is, however, not meant to indicate any hindrance to thedevelopment of new types of coating materials.


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3.2 Ballast water tanksBallast water tanks are generally to be coated with hardprotective coating or an equivalent. Coating system II and IIIare applicable to the additional class notations COAT-1 andCOAT-2.

Coating system II with a target durability of 10 years isregarded as representing a quality level equal to, or above,the intention in SOLAS, Regulation II-1/3-2 [2].

Coating system III with a target durability of 15 years, isregarded as representing a quality level well above theintention in the same SOLAS Regulation.

Coating system I with a target durability of 5 years isregarded as representing a quality level below that intendedin the same SOLAS Regulation, and is not recommended foruse in ballast water tanks.

Light coloured coatings are recommended [2]. The generalviewpoints given under 3.1 are valid for newbuildings.

3.3 Cargo oil tanksCoating systems I, II and III are applicable. Steel platesshould be shop primed, and the primer should contain zincand be silicate based. Coating is recommended in cargo oiltank bottom plating and structures and underneath deck anddeck structures, according to the rules [9].

On the bottom plating, the coating is to prevent bottom-pitting corrosion that can occur in sour, foul water,precipitated from oil cargoes, from condensation of moisturein the air, etc. Bacterial corrosion attacks superimposed oncommon electrolytic corrosion may give very high corrosionrates on bottom plates unless properly protected by a coating.This is especially so in double hull vessels due to the highcargo temperatures being maintained over a prolonged period(thermal isolation or thermos bottle effect of the doublehull).

The topside of horizontal stringer decks should be protectedwith a coating due to its susceptibility to pitting corrosiondue to deposits settling out from the cargo, as for the bottomplating.

Plating and associated structures underneath deck are oftensusceptible to more or less uniform corrosion attacks due tothe cyclic sun heating and cooling effect, watercondensation, abundant oxygen supply and washing andsloshing of cargo. Proper application of a suitable coating isrecommended. Sacrificial anodes will not be effective due tothe fact that there is no bulk electrolyte for the distribution ofthe protective current.

The coating types suitable for cargo oil tanks must be oilresistant and will usually be epoxy based. The coating typeshould be selected in co-operation with the coatingmanufacturer. Coal tar epoxies will usually not be oilresistant and may easily become brittle due to the light tarcomponents that may be washed away and or evaporate, dueto cyclic sun heating.

3.4 Cargo holdsCoating systems I, II and III are generally applicable but maybe modified as regards coating thickness and number ofcoats, if relevant. The coating is to be epoxy or equivalent,giving adequate corrosion protection to the surfaces inquestion. The cargo type and mode of operation of the shipmust also be taken into consideration.

All internal parts of the cargo holds, except flat tank topareas, hopper tanks sloping plating and transverse bulkheadsbottom stool, approximately 300 mm below shell frame, andbrackets are to be coated. Internal and external surfaces ofhatch coamings and hatch covers are also to be coated. Referto the current Rules for Classification of Ships [7], [9].

Coating of the flat tank top areas and other surfacesexempted from the rule requirement [7], [9], see above, isrecommended. This is provided that a coating or floorcovering can be found that is sufficiently tough to withstandthe impacts, abrasion and general rough treatment by grabs,bulldozers, etc. during loading and unloading.

Due concern should be given to coatings used for grain orother edible cargoes that they are properly certified for thepurpose by the responsible authorities.

4. References[1] International Maritime Organization, (IMO). ResolutionA.798(19), Guidelines for the Selection, Application andMaintenance of Corrosion Prevention Systems of DedicatedSeawater Ballast Tanks, adopted 1995.

[2] SOLAS June 1996 Amendments, Regulation II-1/3-2,Corrosion Prevention of Seawater Ballast Tanks, adopted byresolution MSC.47(66).

[3] International Association of Classification Societies(IACS). Guidelines for Acceptance, Application and Surveyof Semi-hard Coatings in Ballast Tanks. IACSRecommendation No. 54, September 1998.

[4] Det Norske Veritas. Type Approval Programme forProtective Coating Systems. No. 1-602.1, 1998.

[5] IACS. Survey Guidelines for Tanks in which SoftCoatings have been applied. IACS Recommendation No. 44,1996.

[6] Det Norske Veritas. Guidelines No. 8, for CorrosionProtection of Ships. (July 1996 or later edition).

[7] IACS Unified Requirement Z9 (1996).

[8] ASTM D 1500. Test Method for ASTM Colour ofPetroleum Products (ASTM Colour Scale).

[9] Det Norske Veritas. Rules for Classification of Ships.(July 1998 or later edition).


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5. Various degree of rusting on paintedsteel surfaces

Figure 1 Examples of degree of rusting on painted steelsurfaces

The black spots of the figure representing 3 % of the surfaceare denoted as Rust Grade 5 and define the limit for coatingdurability (useful life). Those representing 10 % are denotedRust Grade 4. Figure copied from ASTM D610-85.

IntroductionGeneralLimitationDefinitions

Coating SystemsGeneralCoating system no. I Target useful life 5 yearsCoating system no. II Target useful life 10 yeaCoating system no. III - Target useful life 15 years

Specific Applications of Coating SystemsGeneralBallast water tanksCargo oil tanksCargo holds

ReferencesVarious degree of rusting on painted steel surfaces

Documents

Tanks and Hold Corrosion Prevention Dnv