CA1277189C - Protective coating for cathodically protected metal surfaces - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE.

To provide a cover coating for metal surfaces subject to corrosion by water, typically steel surfaces in salt water, for ships, structural components, and the like, in which cathodic protection by application of a negative voltage to the metal is provided, a two-layer coating is applied in which the base coating applied to the metal comprises a mixture of finely divided zinc and polyisocyanate prepolymers capable of hardening or curing under ambient humidity, with dryers, gelling, and pigment agents, and high boiling point solvents, covered by a cover coating, preferably in two to three layers, of a mixture of tar with polyisocyanates, fillers, dryers, solvents, and micaceous iron ore or hematite powder.

Description

" PROTECTIVE COATING FOR CATHODICA:L,LY
PROTE~CTED MET~I, SURFACES "

The present invention relates to a protective coating for cathodically protected surfaces, and more particularly to a coating to protect steel surfaces, such as ship hulls, bridge girder and structural elements and the like which are exposed to water and more particularly to salt water.
sackground and Prior Art. It is well known ~o protect iron and steel structural elements against corrosion by a suitable paint or coating. Structural elements made of iron and steel and submerged in water, parti.cularly hulls of shi.ps and the like, frequerltly are protected not only by ~ paint or similzr coating, that is, by a passive corrosion protection, but aclditionally hy an active corrosion protection by subject.ing the steel plate to an electrical voltage. Cathodic corrosion protection involve.s connecting the s-teel object which i~ to be protected as a cathode with respect to one or more anodes and applying a pro-tecti.ve current thereto, to maintain the article -to be protected at a ~277~

predet~rmined negative protective ~oltage. A suitable range i~ in ~e order of a~out -9S0 to -1150 mV, measured with respect to an ~g/~gCl c~ll.
It has b~en ~ound that customary paint coatings ancl similar coatinys applied on st,eel sur~aces as A protection against corro~ion are darnaged ~ the additional cathodic electrical loacling i~ a combination of paint protection and cal:hodic protection is desired. Customary paint coatings, upon being cathodieally loaded, are prone to form bubbles. In/'one coating which ha3 reeently become available to be applied to steel surface~, a current density in the order of about 50 r~/m2 is required in order to maintain a suitable protective potential thereon. As the age of th~ paint coating increaseq, the current density to maintain the protective potential rises, and may reach values in the order o~ 100 mA/m2 and thereover. As the current density increases, the tendency to form bubbles al.so increases.
The Invention. It is an object to provide a protective coating which is compatible with cathodic corrosion protection arrangements.
Briefly, in accordance with t,he in~errtion, Ihe protective coating whieh is compatible with cathodic protection, partic~ar]y for steel sur~aces, is characterized by a dual~layer coatiny, ln which a base coat is first applied whlch is o~ the zinc powder type and capable of hardenin~ under influence of humidity in a~bien air and having a composition ~ with all parts by weicJht - generally as follows:

g 6-10 parts polyisocyana-te-prepolymer with an NCO content of about 16% and capable of hardening under ambient humidity, 7-9 parts high bolling point aromatic solvent, 0.01-0.03 parts ~eaeration substance, 0.3-0.8 parts dryirlcJ and stabilizing s~hstance, 3-6 parts gelling substance, 0.3-0.5 parts wettiny agent, 4-7 parts pigment i.n flake form and 65-75 parts zinc powder and a cover coating which comprlses a polyurethane-tar coating capable of hardening under ambient air humidity comprising:
16-20 parts aromatic polyisocyanate with an NCO content of 5-7%, 1-3 paxts liquid higher molecular polyisocyanate with an NCO
content oE 3-4~, 5-20 parts tar with a viscosity o~ 5600 to 6400 poiset25C, 6.5-8 parts drying agent, 0.05-0.1 parts deaeration agent, 3.5-4.5 parts precipitation prevention agent, 30-40 parts micaceous i.ron ore or hernat.ite powder, 5-10 parts inoryanic filler,

2-5 parts stabilizing agent al;cl up to 10 parts solvent.

The dual coating system in accordance with the invention does not have a tendency to foxm bubbles even under cathodic loading and, further, has excellent adhesion, a surface which i5 free from pores, high abras~on resistance and hiyh resistance to changin~J due to a~ing. The coating syc;tem preferably inclucles a base coat formed of two individual coatings or layerc and a cover coat formed likewise of two individual coatings or layers, in which each coatin~ or layer of the base coat itself has a thickness of about 75 ~m, and each coating or layer of the cover 10 coating has a thickness of about 125 ~mA steel surface coated in acc~rdance with the system as described can be cathodically protected by applying a voltage in the customary range, but the current flowing is only about 10 mA/m . Thus, the current consumption for cathodic protect.ion against corrosion in accordance with the system of the invention is substantially less than the current consumption of previously coating systems.
The varlous composi.tions used in the coating systems are commerci.ally known and available, and it is their combination arld interaction which provides the surprising effect of freedorn .Erom formation o~ bubbles even when electrically loaded.
In a preEerred form, the surface to be coated .i5 .Eirst sandblasted. The zinc powder base coat is then applied. All parts re;Eerred to hereinafter will be parts by wei~ht.
A b:inder component of 6-10 parts is used, preerably a poly-isocyanate~prepolymer with an ~CO content Oe abou~ 16~, capable of hardening in ambient humidity, for example "Desmodur E 2l"
of the Bayer ~irm. The zinc coating portion further includes 3-6 parts of yel, preferably ln form of a Einely dispersed powder o dimethyl-dioctadecyclammonium-montmorillonite, for example "Bentone 34" of the Titan-Gesellschaft (T.itanium Company), or of an organic derivative of montmorillonite, prepared by lon exchange reaction oE montrnorill.onite with alkylarnmonium bases, parts or flake for example "~entone 38". E'urther, 4-7 /of a scale--lilce/pigment, preferably in forrn of m.icro powder of a naturally ingrown mixture of muscovite, chloxite and quartz with a grinding fineness of 20 micrometers, such as "Plastorit 0000" of ~he Talkurnwerke Nai.ntsch, and 65~75 parts of zinc powder, preferably of a grain size of about 2.5 micrometers. Adcdi-tionally, -the zinc powder coating contains 0.3 to 0.5 parts of a wet-tlng agent based on a salt of a long-chain polyaminamide and a higher molecular acidic ester, such as "Anti-Terra~U" of the firm Byk-Mallinckrodt, 0.3 0.8 parts of a drying and stabilizing ayent, for exampl~ the additives "Ti and OF" of the firm Bayer for polyurethane single-component'paints, 0.01-0.3 parts of a cornmercial deaeration agent, such as "Acronal" of the firm sAsF~ and 7-g% of an - aromatic solvent having a boiling point range in the order of about 160 -180C, for example "Shellsol ~ll by the Sh~l]. company.
A licluid monofunctional isocyanate is clesirably used a~ a cIry~r and ~tabilizing agent since it reacts acti~ely with water and binds the Water quantitatively. The deaeration acJent suitable is a commercial di.spersion or solution on the basls of polyacrylic.acid esters. The zinc powder coatiny with the above cornposition can be applied by brushing on, ro].liny on, compressed air spraying or airless spraying. It i9 app:Lied to a th:ickness steel support surfaee with a layer of a / fabout l50 ~m.
At a relative humidity of 30%, the eoating wi:Ll dry to a cover t~hieh is free of pores and havirlc3 a dry filled th:ickness of about 75 ~m. Th~ zine powdcr eocltiny dries rap.idly, and a:Eter about 20 minutes, a seeond 5ueh coat.ing with a similar clry thiekness of 75 ~rm ean be applied.
The dual layer base eoating is then covered wi~.h a eover eoating whieh is, preferably, also eomposed of two or three layers, and whieh ineludes a polyurethane-tar eover eoating eapable of drying under the in~luenee of ambient humidity. The cover coating, as the main binder eomponent, has 16-20 parts o~ an aromatic polyisoeyanate with an NCO eontent ~ 6-7%, preferably a 60% ~olution o~ ethylglycolacetate/xylol 1 : 1 .in whieh is dissolved an aromotic polyisocyanate with an NCO eontent of about 6.5%, that is, an NCO .
equivalent weiyht of about 646, and available eommercially under the name "Desmodur E 1361" by khe flrm Bayer. The cover coating eontains Euxther binder eornponents, 511Ch as a slow reaet.incJ
liquid higher moleeular polyisoeyanate with an NCO content of

3~4%, preferably a eommereial higher molecular poly~ ocyanate with an NCO eon-tent of about 3.5~ and a vi3co~ity of about 8000 ep at 20C, eomm~reially available under "Ve~modur .E 1ll" of the firm Bayex. The eover eoating further eontains 10--12% tar with a vi~cosity oE 5600 to 6400 poise/25C. Further, it contains 6.S-8 parts o~ dryer, 0.05-1 part deaeration ag~n-t, 3.5-4.5 pa.rts ~27~

precipitation prevention agent, 30-40 parts micaceous iron ore or hematite powder, 5-10 parts inorganic filler, 2.5 parts stabilizer and up to 10 parts solvent. The dryer, typically, is a monofurlctional isocyanate with an NCO content of 21~22~ having a flash point of about 140C, a boiling point Oe a~out 270C and and a hardeniny or solidi~ying point of about 5C, and an index of refraction of 1.53 at 20C, for example the additive "'rI" of the firm Bayer. The dryer binds quantitatively any water which is present and thus co~tributes ko the storage capabilit~ when air is excluded. The deaeration agent preferably is a commercial product which provides for uni~orm deaeration of the cover coating after application, for example "Acronal" of the firm BASF. The precipitation prevention agent preferably is a finely dispersed clay earth, such as aluminum oxide, which prevents separation or loss of mixing of the components of the cover coatiny. The micaceou~
iron ore or hematite powder, basically, has a grain si~e in the range of about 60-75 ~m. The inorganic filler preferably is finel~ divided talcum. ~he stabiliæer preferably is El commercial stabilizer for polyurethane single-component coatincJs which decrease the sensitivity of the cover coating to moisture or humidity and generally improves ~he coating operation, for example t:he additive "OF" of the firm Bayer. The solvent preferEIbly is a mixture of ethylylycolacetate and xylol, in the relationstlip o 1 :
The cover coating, preferably hav.ing two or three i.ndividual layers or coatings, .is then applied over the base zinc coatiny, al60 , ~Z7ql89 .

comprisin~ two layers. The top eoating is applied with a dry film thickness of about 125 ~m per layer. Each one of the applied layers of the top eoatlng will dry or cure in humiclity of about 30æ to a coatlny free from pores which ha.s excellent adhe~ion on the ba~e eoatirly~ The top coating hardens to a protective cover ~ree from pore~ which has excellent chemical and meehanical resistanee.
The particular coating as deseribed has excellent eompatibility with cathodic protection systems and thus i.s partieularly suitable for underwater use for ships, steel structures, and the like.
Example: A sheet of steel, after havinc~ been sandblasted, was eoated with a base eoat of 8 parts o~ commercial, hlImidity hardening polyisocyanate-prepolymer ("Desmodur E 21") with an NCO
content of 16%, 4 parts of commercial aromatic solvent, "Shellsol .
with a boiling point range of from 160 to 180C, 0.02 parts of a eommercial deaerating agent ("Acronal"), 0.5 parts of a commexcial dryer and stabiliz0r for polyurethane single-component paints, the additives "TI" and "OF", 4 parts dimethyldiocta-deeyelammonium~montmoril.lonite, that is, "~entone 3L~", 0.l~ parts of a eommereial w~ttiny agent,"Anti-Terra-U", baæe.cl on a salt o~
a long~ehain polyaminamide and a high moleeular aei.dic e~ter, 6 parts of finely ~round powder oE a rlaturally oecu.rlncJ m.ixture o~ museovite, ehorite and ~uartz, namely "Plastorit 0000", and ~5 70 parts of fine zine powder with an average grain ~ize of 2.5 ~m.

~8~

The drying film thickness of each one of two such base coats.
was 75 ~m.
A cover coating o~ three layers was then applied usiny a coating rr,ixture of 18 parts of commerclal aromal:ic polyisocyanate S with an NCO content of 6.5 in form of a 60% solution in ethyl-glycolacetate/xylol, in a relationship O:e 1 : 1, commercially available as "Desmodur E 1361", 2 parts of commercial li.~uid hiyhe:
molecular poly.isocyanate with an NCO content o~ 3.5%f comrnercially available as "Desmodur E 14", 10 parts tar with a viscosity o~
6000 poise at 25C, 6 parts of dryer, additive "TI", in form of a liquid isocyanate with an NCO content of 21-22X, 0.2 parts of commercial polyvinylether based deaeration agent, known as "Acronal", 3~5 parts of finely dispersed clay, 25 parts micaceous iron ore or hematite powder with a par~icle size in a range of lS 60-75 ~m, 8 parts of microfine talcum, 5 parts ethylglycolacetate 5 parts xylol, and 2 parts of a commercial stabilizer for poly-urethane single-component paints, such as additive "OF".
Each one of the three ].ayers of the top coatincJ had a dxy film thicknes~, of about 150 ~m. The co.-ated sheet steel was suspended in seawater and connectecl over a resistor to a protective anode. A protective potential o~ -l000 mV was app].ied by means o~ a potentiometer. The protective current den.sity to mainta.i.n this voltaye was only about l0 mA/m2 and did not chanye even in the course o~ an experimental period of several months. The coatiny also, a~ter an experimental period of several months, was in excellent condition.

The foregoing examples of the composi-tion show preferred materials and rela-tive proportions. These materials and pro~
portions are not entirely critical, however.
The 6-10 parts polyisocyanate-prepolymer with an NCO
conkent oE a~out 16% can be varied since the NC content can be between about 12 to 20~. If less than the lower limit, for exampl 6% polyisocyanate or about 12% NCO content/ respectively, ls used, adhesion will become poor and the system will,not func~ion as a zinc powder system. Upon increasing the polyisocyanate to about 10~ or about 20~ NCO content, resp~ctively, the metallic contact also will be poor. 16~ is the preferred mean within the and limits/where adhesion is satisfactory.
Rather than using polyisocyanate-prepolymer, which is a one-component material, a two-component epoxy resin comprising a basic component and a curing agent may be used. Such epoxy resins axe commercially available, for example Enertolpoxitar sold by Lechler~Chemie, Stuttgart, Fed. Rep. Germany. This system is not humidity hardening.
The high boiling aromatic solven~ can be replacecl by a low boil:ing aromatic solvent, for example by toluol, xylol, ox a s~table ester. If one uses less than 7~, the solubility becomes insufficient; iE one takes more than 9%, the system will be too thin and liquid. The range o~ ~rom 7% to 9% has been ound ~o be eminently satls~actory.
The deaerating substance, prefexably presen-t in a small quantity of from 0.01% to 0.03% is a special soft resin. One 8~

suitable type is the material sold as "Blister~free" by the Schwegmann company of Bonn, Fed. Rep. Germany. If one takes less than the lower limlt, there will be insufficient deaeration, if too much, the system becomes lumpy and will not dry well or only incompletely.
The drying and stabilizing substance, preEerably, is a poly-isocyanate. The range of between 0.3% to 0.8~ is the optimum less than the lower limit causes thickening of the overall system and difficulty in application, that is, it will not be liquid enough If too much is taken, khe efficiency of the paint as such is not greatly degraded; however, its storage and shelE
life becomes unsatisfactory.
The gelling substance, preferably, is an alumina or argillaceous earth of the kind used in paints of this type; the particular type of gelling substance to be used, and the best amounts with respect to the remairlder of the composition can be easily determined by standard tests well known in this field.
Using either appreciably more or less than ~he given quantity of 3 to 6 parts will detract from appropriate gelling, and result in poor gelling effects.
The preferred wetting agent i5 a polyamide. I~ less than the lower limit is used, the consisteney oE the systein det~riorate~
more than the upper limit wil~. make the system too liquid for usual -type of application.

~27~113~9 The scale-form pigment, preferably, is a mineral filling material in flake form.
The zinc powder preferably has a particle size of between 1 to 20 ~m, with a median value of about 2.5 ~m.
The~ cover coating provides an opaque color. If the NCO
content i~ too low, for example 4% or less, the system would not cure properly; increasing the NCO content much above 7%, for example above 8% or more, may result in health problems. Likewise if the polyisocyanate is substantially decreased or increased, respectively, below 15% and above 21%, either the system would not cure properly or may pose a health hazard.
As with the polyisocyanate for the base coat, a two-compone epoxy resin can be used which, however, does not cure under conditions of humidity.
The higher molecular polyisocyanate, if present with a NCO
content of less than 3%/,makes the system too liquid; if the NCO
content is above 4%, the system would be too hard, particularly when using two-component epoxy resins, and the elasticity of the resulting coating would be impaired.
The range of the tar content may be very wide, 5~-20%;
a range of between 10% to 12%, however, is preferred. IE a lesser amount o~ tar is used,th~-~ system woulcl eEfectively be a no-tar system, with too low water repellency. More than 20% lowers the mechanical stability and softens th~ overal coating too much.

-12~

'g The drying agent, preferably, is a liquid isocyanate monomer. Less than 6.5 parts causes the system to thicken;
more than 8% detracts from shelf life and storage ability.
The relative proportion of the drying ayent - pre~erab:ly within the range of 6.5 - 8 parts will depend, however, on the overall humidity content of the system.
The precipitation prevention agent, if present in excessiv~
quantlty,would result in insufficient hardness; if ~too ~ittle is used, the system suspension will not be maintained and excessive stirring will be needed. A preferred substance is the same as the gelling substance, that is, alumina or argillaceous earth.
The micaceous iron ore or hematite powder can be replaced by other mineral filling substances, for example Plastorit, produced in Aus~ria, and a commercially available substance. Generally, a mixture of SiO~, Al, Mg in form of a dry powder is used.
The inorganic fillers which are preferred are usually di~ferent types of talcum, that is, mainly calcium carhonate and earth alkali salts. A particularly suitable type af talc~ is commercially available under the name of "Norwegian talcum".
The mineral filler and the inorganic filler together cooperate to provide good stability, cohesion and adhesion. Using less than the lower limit given above leads to insuicient stability, cohesion and adhesion. Usiny too much results in insuf~icient binder being present in the remainder of the system, so that the pigments are not bound sufficiently. A roughly inter-, .' : ,' :' : ' ' .
, ~mas mediate range for both the mineral filling sub~stances and khe inorganic filler is preferred, with the overall presence of both rnineral filler and inorganic filler preferably not exceeding ahout 45 parts, overall. ~rhe stabilizing ayent, preferably, i5 a hydrophilic isocyanate. Using l~ss than about 2 parts (b~ weight) leads to instability; usin~ more than 5 parts may result in separation of phases.
The coating preferably uses a solvert, although it is possible to omit the solvent ; using an excess of about 10 parts renders the system too liquid, thus impairing the continuity of the film of the pain-ting system. Any kind of aromatic solvents or esters can be used.
In general, a preferred paint would use approximately the median quantities within the ranges discussed above; the effect of variation of the quantities and relative quantities within the overall system will readily lead to a particular composition suitable for specific applications.

: ` '

Claims (12)

The embodiments of the invention in which an exclusive right or privilege is claimed are defined as follows:

1. Protective coating for cathodically protected surface comprising a base coating and a cover coating, in which the base coating includes a zinc powder base coating capable of hardening or curing under ambient air humidity and having a composition, all parts being approximate and by weight, comprising:
6-10 parts polyisocyanate-prepolymer with an NCO content of about 16% and capable of hardening under ambient humidity, 7-9 parts high boiling point aromatic solvent, 0.01-0.03 parts deaeration substance, 0.3-0.8 parts drying and stabilizing substance, 3-6 parts gelling substance, 0.3-0.5 parts wetting agent, 4-7 parts pigment in flake form and 65-75 parts zinc powder and a cover coating which comprises a polyurethane-tar coating capable of hardening under ambient air humidity comprising:
16-20 parts aromatic polyisocyanate with an NCO content of 5-7%, 1-3 parts liquid higher molecular polyisocyanate with an NCO
content of 3-4%, 5-20 parts tar with a viscosity of 5600 to 6400 poise/25°C, 6.5-8 parts drying agent, 0.05-0.1 parts deaeration agent, 3.5-4.5 parts precipitation prevention agent, 30-40 parts micaceous iron ore or hematite powder, 5-10 parts inorganic filler, and 2-5 parts stabilizing agent.

2. Protective coating for cathodically protected surface comprising a base coating and a cover coating, in which the base coating includes a zinc powder base coating capable of hardening or curing under ambient air humidity and having a composition, all parts being approximate and by weight, comprising:
about 8 parts polyisocyanate-prepolymer with an NCO content of about 16% and capable of hardening under ambient humidity, about 8 parts high boiling point aromatic solvent, about 0.02 parts deaeration substance, about 0.55 parts drying and stabilizing substance, about 4-5 parts gelling substance, about 0.4 parts wetting agent, about 5-6 parts pigment in flake form and about 70 parts zinc powder and a cover coating which comprises a polyurethane-tar coating capable of hardening under ambient air humidity comprising:
about 18 parts aromatic polyisocyanate with an NCO content of 5-7%, about 2 parts liquid higher molecular polyisocyanate with an NCO
content of 3-4%, about 12.5 parts tar with a viscosity of 5600 to 6400 poise/25°C, about 7.3 parts drying agent, about 0.075 parts deaeration agent, about 4 parts precipitation prevention agent, about 35 parts micaceous iron ore or hematite powder, about 7.5 parts inorganic filler, and about 3.5 parts stabilizing agent.

3. Coating according to claim 1 or claim 2, wherein the gelling substance in the zinc-based coating comprises at least one of the materials selected from the group consisting of: dimethyl-dioctadecyclammonium-montmorillonite; an organic derivative of montmorillonite derived from montmorillonite by iron exchange reaction with alkylammonium bases.

4. Coating according to claim 1 or claim 2, wherein the flaked pigment in the zinc-based coating is a finely ground flour of a naturally occurring mixture of muscovite, chlorite and quartz.

5. Coating according to claim 1 or claim 2, wherein the precipitation prevention agent of the cover coating is finely dispersed clay.

6. Coating according to claim 1 or claim 2, wherein the filler in the cover coating is microfine talcum.

7. Coating according to claim 1 or claim 2, wherein the base coating comprises a two-layer coating, each having a dry film thickness of about 75µm, and the top coating comprises at least two layers, each with a dry film thickness of about 125µm.

8. Coating according to claim 1 or claim 2, wherein the tar is present in about 10-12 parts.

9. Coating according to claim 1 or claim 2, wherein the cover coating includes up to about 10 parts solvent.

10. Protective coating for cathodically protected surface comprising a base coating and a cover coating, in which the base coating includes a zinc powder base coating capable of hardening or curing under ambient air humidity and having a composition, all parts being approximate and by weight, comprising:
6-10 parts of an epoxy-type resin, 7-9 parts high boiling point aromatic solvent, 0.01-0.03 parts deaeration substance, 0.3-0.8 parts drying and stabilizing substance, 3-6 parts gelling substance, 0.3-0.5 parts wetting agent, 4-7 parts pigment in flake form and 65-75 parts zinc powder and a cover coating which comprises a polyurethane-tar coating capable of hardening comprising:
16-20 parts of an epoxy-type resin, 1-3 parts liquid higher molecular polyisocyanate with an NCO
content of 3-4%, 5-20 parts tar with a viscosity of 5600 to 6400 poise/25°C, 6.5-8 parts drying agent, 0.05-0.1 parts deaeration agent, 3.5-4.5 parts precipitation prevention agent, 30-40 parts micaceous iron ore or hematite powder, 5-10 parts inorganic filler, and 2-5 parts stabilizing agent.

11. Coating according to claim 10, wherein at least one of the epoxy resins is a two-component resin.

12. Coating according to claim 10, wherein at least one of the epoxy resins is a one-component resin capable of hardening under ambient air humidity condition.