WO2023139564A1 - Anticorrosive coating for metals - Google Patents

Abstract

The present invention relates to a water-based coating for metal surfaces characterized by anticorrosive properties.

Description

"Anticorrosive coating for metals" DESCRIPTION

The corrosive degradation of metal products and equipment threatens not only the industrial production, but also the environmental safety and human health .

There are several studies and research aimed at making coatings for metal surfaces characteri zed by anticorrosive properties .

To counteract the phenomenon of corrosion, the causes can be addressed, making the material less vulnerable through surface protection treatments , capable of forming compact and protective , albeit thin, coatings thereon .

The protection system must achieve the isolation of the product surface from the aggressive external environment and be capable of inhibiting the oxidation processes which give rise to atmospheric corrosion .

Currently, the corrosion protection by means of coating can be of two types :

( a ) application of polymer systems on the metal , involving a surface coating capable of interposing between the material and aggressive species (barrier effect or passive protection) , such as painting (liquid or powder) , for example;

(b) application of metal coatings which, in addition to utilizing the barrier effect, offer electrochemical protection (cathodic or active protection) by preventing oxidation due to a constant supply of electrons (such as hot-dip galvanizing, for example) ;

(c) combined systems, obtained by combining the two previous systems.

Safety regulations are becoming increasingly stringent and in this context it becomes necessary to abandon processes, such as galvanizing, phosphochromatization or application of paints, especially solvent-based, containing dangerous substances, with excellent technological properties but which are highly harmful.

Therefore, the need to develop new protective systems capable of effectively and durably preventing the corrosion of metal surfaces is linked to regulatory, economic, and performance reasons.

The search for alternative products and processes, with low environmental impact and which ensure equal performance levels, while being economically advantageous, has given rise to the development of various technologies , which however have not yet managed to fully establish themselves at the industrial level .

New perspectives have been reached by means of the development of coatings obtained from ormocer ( ORganic Modi fied CERamics ) polymers , also referred to as organic-inorganic "hybrids" , developed by virtue of sol-gel technology .

Selecting suitable precursors , consisting of alkoxides and/or halides of metals or semi -metals , it is possible to develop hydrolysis and condensation reactions to form a colloidal solution of solid particles , of dimensions between 1 nm and 1 pm in a liquid phase , referred to as a sol .

The sol evolves until the formation of a continuous inorganic lattice , containing an interconnected liquid phase , referred to as a gel .

This is typically followed by a heat treatment to eliminate the liquid phase , stabili ze the system, and improve the mechanical properties of the coating .

The use of hybrid precursors , characteri zed by an organic functionality ( for example , based on epoxy oligomers ) allows overcoming the drawbacks in terms of flexibility and adhesion of the surface film and control of the final features , otherwise conditioned by an exclusively inorganic lattice , consisting of the nanodomains of metals or semi-metals .

In addition, the sol-gel technique also has the possibility to introduce dopants into the developed network to increase speci fic properties .

As shown by the research published by Gao et al . (Xiang Gao , Ru Yan, Lei Xu, Houyi Ma . "Ef fect of amorphous phytic acid nanoparticles on the corrosion mitigation performance and stability of sol-gel coatings on cold-rolled steel substrates" . Journal of Alloys and Compounds ( 2018 ) ) , the anticorrosive activity of alkoxysilane-based sol-gel coatings in the presence of phytic acid 4-s — signi ficant , both in terms of mitigation of the corrosive process and in terms of solution stability . However, the authors have not demonstrated the suitability of the coatings to withstand mechanical stresses at high temperatures such as those encountered by semi- finished metal products during the most common production cycles . In fact , as a function of the operating conditions , creeping, corrosion, oxidation, and surface erosion phenomena can often be triggered on such surfaces .

Despite many attempts , to date there are still no available commercial products capable of minimi zing or preventing the formation of cracks and the detachment of the protective film following the thermal expansion of the metal material .

The prior-art document EP 3 , 398 , 998 describes the preparation of a zinc dust paint comprising an epoxysilane component represented by glycidyloxypropyl alkoxysilane ( GLYMO) .

The prior-art document WO 2015/ 067776 describes the preparation of an anticorrosive coating against the corrosion produced by microbes having a thickness of at least 50 pm .

The prior-art document WO 2010/ 112481 describes an anticorrosive mercaptosilane-based preparation for solar panels to obtain a protective layer having a thickness of at least 0 . 5-500 nm .

Summary of the invention

The present inventors have surprisingly found how to prepare a coating for metal surfaces , characteri zed by improved adhesion properties of the coating and resistance to corrosive phenomena, while being producible by a simple process and adopting mild process operating conditions .

Obj ect of the invention

In a first obj ect , the invention describes a water-based formulation having anticorrosive properties for metal surfaces . In a second obj ect , the present invention describes a process for preparing a metal surface having anticorrosive properties .

In a third obj ect , a method for imparting anticorrosive properties to metal surfaces is described .

Metal surfaces and metal products comprising the obtained surfaces having anticorrosive properties represent further obj ects of the invention .

Detailed description of the invention

In an obj ect of the invention, a process for preparing a metal surface having anticorrosive properties is described .

In particular, such a process comprises the preparation of a formulation to be applied as a coating to said surface .

For the purposes of the present invention, the term " formulation" is intended as a preparation obtained by a sol-gel process .

The term " sol-gel process" means the hydrolysis and condensation reactions originating from alkoxide or inorganic precursors , under appropriate operating conditions , to form glassy networks with micro- or nano-porosity features . For the purposes of the present invention, the formulation is water based; therefore, it is not produced using organic solvents, such as alcohols.

The dry residue of the formulation of the invention is less than 10% or even <8% (weight) .

For the purposes of the present invention, the formulation has a silane-derived inorganic component as an active ingredient.

More in particular, the formulation of the invention comprises three main components: a component represented by one or more sol-gel precursors (sol-gel component) , a component of organic nature to improve the cross-linking of the film, and a component to reduce the surface energy of the treated surface, thus reducing the wettability thereof .

As for the sol-gel component, this can comprise one or more precursors from: alkylsilanes, aminosilanes, epoxysilanes or hydroxysilanes, where the aminosilane precursor is preferred.

In a particular aspect, the alkylsilane is selected from the group comprising: methyltrimethoxysilane, methyltriethoxysilane, and ethyl trimethoxysilane, ethyl triethoxysilane, propyl trimethoxysilane, propyl triethoxysilane, octyl trimethoxysilane, octyl triethoxysilane, dodecyltrimethoxysilane, dodecyltriethoxysilane .

In a particular aspect, the aminosilane is selected from the group comprising: ( 3-aminopropyl ) - trimethoxysilane (APTMS) , ( 3-aminopropyl ) - triethoxysilane (APTES) , ( 3-aminopropyl ) -diethoxymethylsilane (APDEMS) , ( 3-aminopropyl ) -dimethoxymethylsilane (APDMMS) , ( 3-aminopropyl ) -diethoxymethylsilane (APDEES) , ( 3-aminopropyl ) -dimethoxyethylsilane (APDMES) , aminopropyl-terminated polydimethylsiloxane (APT-PDMS) , aminopropyl- terminated polydiethylsiloxane (APT-PDES) , and aminopropylmethylsiloxane-dimethylsiloxane (APM-DMS) .

Preferably, the aminosilane is represented by ( 3-aminopropyl ) -triethoxysilane (APTES) .

According to the present invention, only one type of sol-gel component among those listed or a mixture thereof can be used to enjoy the possible synergy thereof.

For the purposes of the present invention, during the sol-gel process, an acidic or basic catalyst can be used; for example, hydrochloric acid, nitric acid, phosphoric acid, sodium hydroxide, or potassium hydroxide can be used. In a preferred aspect, the catalyst is acidic and, even more preferably, is represented by phosphoric acid.

As for the organic component, which improves the cross-linking of the film, this can be represented, for example, by: acrylic resins, polyurethane compounds (of mono- or bi-component type) , or isocyanates .

As for the polyurethane compounds, these can comprise, for example: toluene diisocyanate and/or methylenediphenyl diisocyanate.

In a preferred aspect of the invention, such a component is represented by isocyanate, preferably aliphatic .

As for the component useful for reducing the surface energy of the surfaces, this can be represented by one or by a mixture of fluorinated compounds in the form of monomers or polymers of different nature, including acrylic compounds, methacrylates, and C4 and C6-based polyurethanes.

In a preferred aspect, such components are free of PFOS (perfluorooctanesulfonic acid) and/or PFOA (perf luorooctanoic acid) or substances belonging to ANNEX XVII of Regulation (EC) No. 1907/2006, known as REACH (Registration, Evaluation and Authorisation of Chemicals) as amended by the publication of Commission Regulation (EU) dd. December 13, 2021, No. 2021/2204 on the G.U.C.E. Law dd. December 14, 2021, No. 446.

As an alternative to fluorinated compounds (or fluorocarbon substances) , it is possible to use compounds characterized by saturated and unsaturated C8-C24 hydrocarbon chains, having carboxylic, amino end groups .

Such substances can comprise, for example: octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, nonadecanoic acid, eicosanoic acid, docosanoic acid, tetracosanoic acid, cis-9-hexadecenoic acid, cis-10- heptadecenoic acid, cis-9-octadecenoic acid, trans-9- octadecenoic acid, trans-ll-octadecenoic acid, cis- 11-octadecenoic acid, cis- 6-octadecenoic acid, trans- 6-octadecenoic acid, cis-9-eicosenoic acid, cis-11- eicosenoic acid, cis-ll-docosenoic acid, cis-13- docosenoic acid, cis-15-tetracosenoic acid, 9, 12- octadecadienoic acid, 9, 12 , 15-octadecatrienoic acid, 6, 9, 12-octadecatrienoic acid, 6, 9,12,15- octadecatetraenoic acid, 5, 8, 11, 14-eicosatetraenoic acid, 5, 8, 11, 14, 17-eicosapentaenoic acid, 7, 10, 13, 16, 19-docosapentaenoic acid, octylamine, decylamine, dodecylamine, octadecylamine, decyldimethylamine, 2-n-octyl-l-dodecylamine .

Such a component confers water-repellent properties to the surfaces to which it is applied, contributing to increase the anticorrosive effect.

Such a component further reduces surface friction, being particularly useful if the formulation of the invention is applied to strands or metal surfaces in contact or sliding with each other.

As described above, the solvent is represented by water.

The formulation described by the present patent application can further comprise further components, referred to as "secondary components".

Such secondary components include dopants, emulsifiers and sequestrants ; moreover, a compound having antifermentation activity can be added.

As for dopants, these can be represented, for example, by: inorganic particles such as kaolin, titanium oxide, calcium magnesium carbonate, silicon oxide, and derivatives.

Such agents can give the formulation of the invention greater resistance to wear or even corrosion; by virtue of the porosity thereof, they can be loaded acting as carriers for the release of appropriate agents under specific conditions (e.g., humidity or pH) .

As for the emulsifiers, these can be represented, for example, by: fatty acid alcohols (e.g., C12-C14 laurylmyristyl alcohol, oleic alcohol, behenyl alcohol) , ethoxylated alcohols (e.g., ethoxylated cetylstearyl alcohols, ethoxylated oleic alcohols, cetyloleic alcohols, ethoxylated C12-14 alcohols, ethoxylated stearyl alcohols, ethoxylated isodecyl alcohols) , PEG and sorbitan esters, and polysorbates .

As for the sequestrants , these can be represented, for example, by phosphorus derivatives.

Sequestrants and emulsifiers are used to decrease the hardness of the water used and/or to stabilize the formulation, understood as a combination of the main and secondary components.

The process for preparing the formulation according to the present invention comprises the steps of:

1) preparing an aqueous solution of the sol-gel component, possibly in the presence of a catalyst, sequestrants, and emulsifiers; 2 ) adding the component to reduce the surface energy of the treated surface and the organic component used to improve the cross-linking of the film .

For the purposes of the present invention, step 1 ) comprises adding the sol-gel components to the aqueous solution in an amount of about 1-20% (w/w) , and more preferably about 10% (w/w) .

In step 1 ) , the possible addition of the active ingredient to the aqueous solution is carried out in the presence of a catalyst .

In an embodiment of the invention, the waterbased formulation comprises aminopropyltriethoxysilane (APTES ) as a sol-gel precursor .

For the preparation of the formulation of the invention, the sol-gel precursor ( s ) is added to the aqueous solution under stirring .

The stirring aims to obtain a homogeneous dispersion of the sol-gel precursor within the solution .

More in particular, the stirring is carried out at the speed of about 200- 1500 rpm and preferably about 1000 rpm . More in particular, the stirring is maintained for a time period of 1-240 minutes and preferably about 20 minutes .

More in particular, room temperature ( 25 ° C ) is maintained during the stirring .

Once the stirring step is completed, the surface tension-reducing component and the organic component are added to the formulation to improve the crosslinking of the film .

As for the other components , such as dopants , emulsi fiers , sequestrants , these are added individually in concentrations generally between about 0 . 2-5 (w/w) under stirring and can be added in the formulation after the addition of the sol-gel component or preferably before .

According to an aspect of the invention, the formulation thus obtained has a pH value ranging from 4 to 11 .

The present invention describes the preparation of a surface with anticorrosive properties ; such surfaces are obtained by applying the formulation of the invention detailed above to said surface and allowing it to dry . The term "anticorrosive" means the ability to inhibit and/or decrease and/or limit metal corrosion phenomena .

More in particular, reference is made to all the possible forms of corrosion, including: uniform corrosion (surface alveolar corrosion) , pitting corrosion, crevice corrosion, environmental embrittlement, intercrystalline corrosion (intergranular) , galvanic corrosion (contact) , stress corrosion (stress cracking) .

For the purposes of the present patent application, a surface treated with the formulation of the invention is to be understood as a metal surface .

Metal surfaces include metal surfaces such as in iron, copper, aluminum, or metal alloy surfaces, such as steel and brass.

For the purposes of the present invention, metal surfaces as such or coated, for example by painting, are included.

Particular examples of surfaces include products and surfaces such as: metal cables, braided metal cables (singly or collectively) , strands (singly or collectively) , ropes, reinforcements and reinforcement elements for reinforced concrete, reinforcements and reinforcement elements for prestressed reinforced concrete .

For the preparation of the surfaces having anticorrosive activity according to the invention, the formulation prepared as described above is applied to a surface .

For the purposes of the present invention, such a surface can be painted and, therefore , the formulation of the invention is applied above the layer of paint .

In a preferred aspect of the invention, the surface to be treated must be thoroughly cleaned prior to application .

For this purpose , the surface , even painted, can be cleaned for example by means of alcohols , water, and detergents , by pickling or sandblasting .

Optionally, the cleaning step is followed by a rinsing step with softened, osmotic, or distilled water .

In any case , even in the absence of cleaning, the formulation of the invention has demonstrated excellent adhesion properties and anticorrosive capabilities .

As for the application mode , the formulation can be applied, for example , by : - brush,

- roller,

- spray,

- dip-coating,

- sponging .

According to a first aspect of the invention, the formulation can be applied as such, while in an alternative aspect , it can be appropriately diluted .

For example , the dilution can meet requirements due to the application technique or the type of surface to be treated .

The application is carried out for a period of time ranging from a few seconds to a few minutes as a function of the application mode ( continuous or discontinuous ) .

The application is preferably carried out at room temperature and therefore advantageously does not require a heating system for the application bath .

The application step is followed by a drying step .

To this end, the drying can be carried out at a temperature of about 10-220 ° C, preferably 20- 100 ° C or it can be carried out in an oven to accelerate the aforesaid drying procedures , using variable temperatures as a function of the exposure time .

Depending on the temperature , drying is carried out for an appropriate period of time , ranging from a few instants to a few days .

The drying conditions depend on the thickness of the applied coating, the nature of the surface , and the humidity conditions .

By way of example , a time period of 24-72 hours at 30 ° C with a humidity of 65% RH can be necessary for deep drying .

The use of an oven allows drying with a duration ranging from a few instants to a few hours , as a function of the si ze of the oven .

Using an induction system, the drying times can be reduced to a few seconds or fraction of a second .

For the purposes of the present invention, the application can be carried out in single layer or in successive layers , after the previous layer has completely dried .

The formulation of the present invention allows obtaining a final thickness less than 50 pm .

The surfaces obtained according to the above description are further obj ects of the present invention . In particular, these are the surfaces obtained after the application and drying steps, possibly preceded by cleaning.

The present invention further describes a method for imparting anticorrosive properties to a surface according to the process described above.

In particular, such anticorrosive properties also remain following surface damage.

The anticorrosive property for structural objects is determined according to UNI EN 1090-2.

Stress corrosion tests were conducted according to UNI EN ISO 15630-3.

According to a particular aspect of the invention, the method proves effective, with anticorrosive activity against different forms of corrosion, such as: uniform corrosion (surface alveolar corrosion) , pitting corrosion, crevice corrosion, environmental embrittlement, intercrystalline corrosion (intergranular) , galvanic corrosion (contact) , stress corrosion (stress cracking) . Results :

The treated material , subj ected to stress corrosion tests according to UNI EN ISO 15630-3 , has a longer li fe than the untreated material , equal to the number of times reported in the table above .

The present invention is hereinafter better described by way of some explanatory and non-limiting examples .

Example 1

Preparation of surfaces having anticorrosive properties

Step 1 - preparation

Under constant stirring, 2 g aminopropyl-terminated polydimethylsiloxane was added in water to form an aqueous solution . To this solution 4 g aqueous HC1 solution were added, in the presence of 0 . 2 g sodium hexametaphosphate , 3 g fluorocarbon resin, 1 . 5 g isocyanate and an anti fermentation agent .

The solution with final volume 70 ml was kept under stirring for about 20 min before application .

Step 2 - application .

The solution prepared in Step 1 was applied to a metal material by dip coating .

Step 3 - Drying/curing . The treated polymer material was dried for 48 hours at a temperature of 35°C before use.

Example 2

Step 1 - preparation

Under constant stirring, 3 g APTES was added in water to form an aqueous solution. To this solution 0.1 g H3PO4 (0.1 M) , 0.2 g sodium hexametaphosphate, 1.5 g isocyanate, 10 g a fluorocarbon resin and an antifermentation agent were added.

The solution with final volume 100 ml was kept under stirring for about 30 minutes before application.

Step 2 - application.

The solution prepared in Step 1 was applied to a metal material by spray application.

Step 3 - Drying/curing .

The treated polymer material was dried for 1 second at a temperature of 200°C before use.

From reading the description of the present invention provided above, the several advantages offered will become apparent to those skilled in the art .

In particular, the described surface coating obtained by applying the formulation of the invention is capable of imparting anticorrosive properties. By virtue of excellent adhesion capabilities , the coating obtained by the formulation of the present invention is capable of of fering such properties for a long period of time , resisting to use and wear and especially damage to the coating itsel f .

The formulation of the invention allows the formation of a coating which is however flexible , an important feature for application to metal surfaces or metal cables or strands ; thereby, in fact , the coating adapts to the elastic movements of the treated surfaces , avoiding the formation of cracks and crevices , which can lead to the exposure of underlying metal parts .

Furthermore , the coating is UV resistant .

Furthermore , the formulation is water-based and chemical agents with a high environmental impact are therefore excluded from the composition, such as : formaldehyde , APEO, PFOA and PFOS , oxime , halogenated and/or aromatic solvents , benzenes and chlorinated toluenes , chlorophenols , hexavalent chromium, tin- based compounds , arylamines , etc .

The process for preparing the formulation and applying it to the surfaces of interest is however simple and practical to implement . For example , the process described does not require a heating system for the application bath, which is kept at room temperature , and requires application times ranging from a few seconds to a few minutes .

Furthermore , the system is robust and tolerates a wide range of pH : from 4 to 11 .

Further advantages are combined with those mentioned above due to the absence of pre-treatment steps of the material and the wide flexibility, by virtue of the possibility to apply the preparation by dipping, spraying or roll coating .

Furthermore , the new process can be fully integrated into the processes already used, thus not requiring the construction of new and speci fic plants .

From the environmental point of view, the process described responds to the current needs of industrial productions with greater respect for human health and the community, as well as the environment .

In order to meet contingent and speci fic needs , those skilled in the art can make adaptations and modi fications to the invention as described above , and can replace elements with others which are functionally equivalent ones , without however departing from the scope of the following claims .

Claims

1. A water-based formulation having anticorrosive properties comprising:

- a component represented by one or more sol-gel precursors selected from the precursors: alkylsilanes, aminosilanes, epoxysilanes or hydroxysilanes , a component of organic nature to improve the cross-linking of the film, and a component to reduce the surface energy of the treated surface, thus reducing the wettability thereof .

2. A water-based formulation having anticorrosive properties according to claim 1, wherein said sol-gel precursors are aminosilane precursors.

3. A water-based formulation having anticorrosive properties according to claim 1 or 2, wherein said precursor is selected from the group comprising: (3- aminopropyl ) -trimethoxysilane (APTMS) , (3- aminopropyl ) -triethoxysilane (APTES) , (3- aminopropyl ) -diethoxy-methylsilane (APDEMS) , (3- aminopropyl ) -dimethoxy-methylsilane (APDMMS) , (3- aminopropyl ) -diethoxy-methylsilane (APDEES) , (3- aminopropyl ) -dimethoxy-ethylsilane (APDMES) , aminopropyl-terminated polydimethylsiloxane (APT- PDMS ) , aminopropyl-terminated polydiethylsiloxane (APT-PDES ) , and aminopropylmethylsiloxanedimethylsiloxane (APM-DMS ) .

4 . A water-based formulation having anticorrosive properties according to any one of the preceding claims , wherein said sol-gel precursor is aminopropyltriethoxysilane (APTES ) .

5 . A water-based formulation having anticorrosive properties according to any one of the preceding claims , wherein said organic component comprises polyurethane compounds selected from toluene diisocyanate and/or methylenediphenyl diisocyanate .

6. A water-based formulation having anticorrosive properties according to any one of the preceding claims , wherein said component capable of reducing the wettability is represented : by one or a mixture of fluorinated compounds in the form of monomers or polymers of di f ferent nature , including C4 and C6-based acrylic, methacrylate and polyurethane compounds ; or by compounds characteri zed by saturated and unsaturated C8-C24 hydrocarbon chains , having carboxylic, amino end groups .

7 . A water-based formulation having anticorrosive properties according to any one of the preceding claims, further comprising one or more of the agents selected from: dopants, emulsifiers, and sequestrants .

8. A water-based formulation having anticorrosive properties according to any one of the preceding claims, further comprising a catalyst.

9. A water-based formulation having anticorrosive properties according to the preceding claim, wherein said catalyst is selected from the group comprising: hydrochloric acid, nitric acid, phosphoric acid, sodium hydroxide or potassium hydroxide and is preferably represented by phosphoric acid.

10. A water-based formulation having anticorrosive properties according to claim 7, wherein said dopant is selected from the group comprising: inorganic particles such as kaolin, titanium oxide, calcium and magnesium carbonate, silicon oxide, and derivatives.

11. A water-based formulation having anti-corrosive properties according to claim 7, wherein said emulsifier is selected from the group comprising: fatty acid alcohols such as C12-C14 lauryl myristyl alcohol, oleic alcohol, behenyl alcohol, ethoxylated alcohols such as ethoxylated cetylstearyl alcohols, ethoxylated oleic alcohols, cetyl oleic alcohols, ethoxylated C12-14 alcohols, ethoxylated stearyl alcohols, ethoxylated isodecyl alcohols, PEG and sorbitan esters, and polysorbates.

12. A process for preparing a metal surface having anticorrosive properties, comprising the steps of: preparing a water-based formulation according to any one of the preceding claims; applying such a formulation to said surface ; allowing the formulation to dry on the surface .

13. A process according to the preceding claim, wherein said surface is painted.

14. A process according to any one of the preceding claims, wherein said surface, possibly painted, is washed before the application of the formulation, by means of alcohols, water and detergents, by pickling or sandblasting.

15. A process according to any one of the preceding claims, wherein said formulation is applied to said surface by:

- brush,

- roller,

- spray,

- dip-coating,

- sponging.

16. A process according to any one of claims 12 to 15 , wherein said formulation is applied to the surface at room temperature .

17 . A surface obtained with the process according to any one of the preceding claims 12 to 16 .

18 . A surface according to the preceding claim, which is the surface of metal cables , braided metal cables , strands , ropes , reinforcements and reinforcement elements for reinforced concrete , reinforcements or reinforcement elements for prestressed reinforced concrete .

19. A method for imparting anticorrosive properties to metal surfaces comprising treating such surfaces with a formulation according to any one of the preceding claims 1 to 11 .

20 . A method according to the preceding claim, wherein said anticorrosive activity is against a form of corrosion selected from the group comprising : uni form corrosion, pitting corrosion, crevice corrosion, environmental embrittlement , intercrystalline corrosion, galvanic ( contact ) corrosion, stress corrosion .

21 . A method according to claim 19 or 20 , wherein such properties remain even after damage to the surface .