DESCRIPTION

The present invention relates to a process for the anti-corrosion coating of small metal parts, capable of giving the small metal parts adequate corrosion resistance characteristics and capable of standardizing the friction values of the various components of the small metal parts treated.

Currently, to protect small metal parts, i.e., screws, nuts, helical inserts, washers, circlips, pins and other metal fastening systems, from possible corrosion actions caused by dampness in the air during storage and use, they are treated with processes that are somewhat laborious and costly, or are incompatible with human health and the protection of the environment.

The term corrosion indicates a slow and continuous process of aggression and wear of a material, resulting in deterioration of the physical characteristics or properties of this material. To evaluate the corrosion resistance of small metal parts, some tests are carried out in accelerated corrosion conditions, in order to evaluate their resistance in a short time. During corrosion tests, the material is subjected to specific conditions which should cause their corrosion. The effects of these conditions are measured after or during the test.

An example of corrosion test is the salt spray chamber, which is an accelerated technical corrosion test for the qualitative evaluation of the corrosion resistance of a material, or of the corrosion protection provided by a coating. This test uses an apparatus inside which the material to be tested is subjected to an environment with a high salt concentration, so as to simulate the behavior of the material in marine environments. This type of method is regulated by the standard ISO 9227, having as title: Corrosion tests in artificial atmospheres — Salt spray tests.

The most common processes for the treatment of small metal parts currently used in order to reduce corrosion are phosphating, electrogalvanizing and coating with resins containing metal flakes. The process according to the invention proposes replacing phosphating.

Phosphating is a chemical process by means of which the surface of a metal material is altered, creating phosphate crystals chemically bonded to the substrate with the aim of exploiting the properties of these compounds to improve corrosion resistance and possibly promote the adhesion of a paint applied subsequently. The phosphating treatment uses metal phosphate solutions (zinc, iron, manganese, nickel phosphates) to coat the surface of the part to be treated with a protective crystalline layer with a phosphate thickness of around 5 - 10 μιη, which provides corrosion resistance and improves adhesion of the subsequent layer of paint through an increase in microscopic surface roughness. The phosphating process is carried out in immersion plants, normally integrated with the degreasing plants. A typical example of phosphating cycle for metal materials such as small metal parts comprises the following steps:

A. Degreasing (de-oiling) to remove residues of oily compounds from manufacture (e.g.: heat treatment tempering oil).

B. Acid pickling, typically by the use of sulfuric or hydrochloric acid. This step is universally known for including hydrogen in the metals, in particular if these are high- alloy metals. The inclusion of hydrogen can produce delayed cracking, pitting and fissures.

C. Phosphating. Actual phosphating treatment is a chemical surface transformation process by means of zinc, manganese, iron and/or calcium phosphates. During the reaction, dissolution of the base material (Fe) takes place, contributing to the formation of the phosphate layer. The surface layer of the base metal thus influences the crystalline structure of the coating and the final appearance. The coating consists of a dense layer of minute crystals, which adhere strongly to the substrate. The thickness is approximately from 5 to 11 μιη (corresponding to 8 - 15 g/m ² ). Due to the porous structure, the phosphate coating alone has limited anti-corrosion characteristics.

D. Oiling. To increase the protection efficacy, the phosphate layer is impregnated with emulsified or neat oils. According to the products used, this operation has an anti- corrosion function, usually aimed at allowing the treated parts to travel through a short logistics path between production and use without being damaged. Final oiling should also allow a given friction coefficient to be obtained, at least until use. This characteristic is essential, for example, in engine screws where the tension installed through tightening must be specific and repeatable. In fact, the oil applied tends to disperse during the logistics path (transport, selection, packaging and handling) generating loss of and/or nonconformity with the required corrosion resistance and friction coefficient characteristics.

It is therefore evident that the phosphating process is very complex, requiring, as well as degreasing, acid pickling, the actual phosphating operation and oiling, as well as various other steps such as rinsing, activation, to promote even crystal distribution, and passivation, which eliminates any crystals of different type, which would cause the paint to detach in a damp environment. The phosphating plant is therefore complex and requires, as well as large investments for its implementation, high operating costs for the consumption of chemical products, water and energy. Moreover, it requires careful management, with frequent analysis of the baths and consequent restoring of optimal conditions, removal of the sludges produced in the phosphating tank and in the degreasing tanks, and management of the acid baths.

The sludges that are produced in the phosphating bath are, in particular, harmful for the environment and for human health. These sludges must be disposed of correctly in order to prevent any contamination of the environment and of the operators.

All operators in the pipeline (nut and bolt manufacturers, phosphating process operators and final users) are subjected to the disadvantages of phosphating, which has continued to be used due to the lack of valid alternatives. The following table summarizes the drawbacks linked to phosphating:

Operators Nut and bolt manufacturers Users

Polluting procedure, requires Fragmentation of production Inconsistency of the substantial investments to batches to minimize the impact friction coefficient with comply with legislation on of nonconforming batches risk of incorrect the working environment tightening

Sludge disposal costs Pollution of the selection Delayed cracking on

machines vehicles in the network

Warranty costs for hydrogen Pollution of the packaging lines Risk of recall campaigns embrittlement

Requires dehydrogenation Warranty costs for delayed Receipt of product plants and operations cracking due to hydrogen batches corroded before without any guarantee of embrittlement assembly

outcome

Warranty costs for lack of Warranty costs for lack of Onerous management of corrosion resistance corrosion resistance nonconformities from

suppliers

Warranty costs for lack of Warranty costs for lack of Pollution of the automatic compliance with the friction compliance with the friction feed lines

coefficient coefficient

Requires dedicated lines Decrease in available suppliers Pollution of the

tightening means

Constant increase in the Requires incoming checks for Potential union

costs critical parts complaints for pollution of the workplace US 2004/0127625 Al describes a coating composition for use in the protection of metal surfaces, which comprise a binder, a corrosion inhibitor, and a solvent. The corrosion inhibitor comprises aluminum particles and zinc particles. It can also comprise a metal phosphate as an anti-corrosion additive. The solvent is an organic solvent, preferably a hydrocarbon solvent. This coating composition belongs to the category of coatings containing metal flakes, which are expensive, and may contain phosphates, in which case it does not avoid the disadvantages of the phosphating process mentioned above. Coatings containing metal flakes may release in the environment metal particles in the form of powders. These powders have a negative impact on the environment, do not comply to the cleanliness standards and are not compatible with the liquids present in an engine or in a vehicle, since they could contaminate such liquids. Furthermore, the coating composition of US 2004/0127625 Al contains organic solvents, which are undesirable for reasons well known to a skilled person, including release in the environment of volatile organic compounds (VOC). US 2012/0237778 Al describes aqueous fiuoropolymer coating compositions and coated metal substrates with improved corrosion resistance. The metal substrate is typically pre- treated with phosphates, so that when the resin composition is applied, the metal is already coated. An organic solvent may be added to aid in the film formation. Therefore, the same disadvantages pointed out for US 2004/0127625 Al apply.

It would therefore be desirable to provide a process for the anti-corrosion coating of small metal parts that is able to produce a high quality product, i.e., a product that satisfies the main corrosion evaluation tests, such as the salt spray chamber test, according to the required technical specifications.

It would also be desirable to provide a process for the coating of small metal parts with no or low environmental pollution, which is not harmful for humans, generates little or no processing waste and is economically advantageous. In particular, a process that does not comprise use of phosphates and of organic solvents, but also a process that does not produce dust when the parts are moved.

Moreover, it would be desirable to provide a process for the coating of small metal parts capable of produce a product with a friction coefficient having a variability range that is limited and as constant as possible, so that the small metal parts treated with this process can be tightened uniformly without a wide difference in tightening torque between one part and the next.

Therefore, an object of the present invention is to provide a process for the coating of small metal parts that is capable of giving corrosion resistance characteristics superior to phosphating, in particular that is capable of giving sufficient salt spray resistance for the use of these small metal parts in the motor vehicle industry assembly line.

A further object of the present invention is to provide a process for the coating of small metal parts that is not polluting, i.e., that does not use substances harmful for the environment and for humans, and that does not produce toxic processing waste that must be further treated to be disposed of.

Yet another object of the invention is to provide a process for the coating of small metal parts requiring a surface preparation of the small metal parts that does not cause embrittlement due to hydrogen inclusion, preventing risks of fatigue failure and/or delayed cracking.

Yet another object of the present invention is to provide a process for the coating of small metal parts capable of supplying a product with a standardized friction coefficient, i.e., the various components of the small metal parts have a similar friction coefficient.

Still another object of the present invention is to provide a process for the coating of small metal parts that is cheaper than the processes that apply aluminum particles and zinc particles. The aforesaid and other objects and advantages of the present invention, which will be apparent from the description below, are achieved by means of a process for the anti- corrosion coating of uncoated small metal parts comprising the following steps:

A. Surface preparation of the uncoated small metal parts, comprising the steps of:

Al . Degreasing, carried out with alkali solutions containing surfactants to remove dirt and oil residues;

A2. Sandblasting, carried out in a shotblasting machine with grit of a size from 50 to 300 micrometers for a sandblasting time variable from 3 to 12 minutes;

A3. Dust collection, followed by drying and air cooling to bring the uncoated small metal parts to ambient temperature;

B. Application of an anti-corrosion coating, consisting of immersing the container containing the uncoated small metal parts in an aqueous dispersion which is free of organic solvents, comprising:

i. a resin selected from the group consisting of epoxy resins, melamine resins, acrylic resins and phenolic resins;

ii. a hardener for the selected resin;

iii. a catalyst for the selected resin;

iv. a lubricant;

for an immersion time from 10 to 60 seconds and the formation of a coating layer on the small metal parts having a thickness of 4 to 15 micrometers; C. Centrifugation of the coated small metal parts in order to remove excess dispersion, carried out in a centrifuge at a rotation speed from 100 to 600 revolutions/minute and at an inclination from 0 to 60°;

D. Hardening of the resin that forms the coating of the coated small metal parts at a temperature from 110 to 320°C, carried out in an oven for a time from 10 to 45 minutes.

The process defined above represents a valid alternative to the phosphating process and has none of its drawbacks. It is described more in detail hereunder, also with reference to the accompanying figures, wherein:

Fig. 1 is a general layout of an embodiment of the process according to the invention;

Fig. 2 is a photograph of a shotblasting apparatus for the implementation of step A2 of the process according to the invention;

Fig. 3 is a photograph of centrifuge for the implementation of step D of the process according to the invention;

Fig. 4 is a photograph of an oven for the implementation of step E of the process according to the invention.

In the present description the term "uncoated metal parts" means metal parts that have not been subjected to any previous anti-corrosion treatment. As stated above, the process for the anti-corrosion coating according to the invention is aimed at replacing in particular the phosphating process. Therefore, the process according to the invention is carried out on uncoated metal parts, particularly on metal parts that that have not been subjected to a phosphating process, but also that have not been subjected to any other anti-corrosion process such as electro galvanizing or coating processes applying metal flakes.

Steps A1-A3 are physical treatments carried out prior to application of the anti-corrosion coating. They are required to clean the metal parts so as to make them suitable and ready for application of the anti-corrosion coating.

In a preferred embodiment, the amount of small metal parts to be treated varies between 20 and 350 kg, as a function both of the characteristics of the treatment line and of the geometry of the treated parts. Advantageously, the "dip-spin" line, normally used for the application of zinc flakes, as is known to the person skilled in the art, can be used. As a rule, the parts are loaded into perforated stainless-steel baskets of adequate capacity. A general layout of a dip- spin line is shown in Fig. 1.

The surface preparation treatments start with a degreasing step Al, which is carried out in order to remove oil and dirt residues on the metal surface, which can prevent correct adhesion of the coating to be applied to the small metal parts. Typically, degreasing on metal products is carried out by means of alkali solutions containing suitable surfactants, which are capable of removing both dirt and oil residues. No organic solvents are used for this degreasing step. Said step takes place through immersion of the small metal parts in tanks containing the alkali degreaser and any other components used to remove oil particles.

The step A2 of sandblasting in a shotblasting machine, shown in Fig. 2, is carried out using grit of a size from 50 to 300 micrometers, consisting of a material such as stainless steel, iron or alloys, of suitable shape for the characteristics and geometry of the products. The sandblasting time can vary from 3 to 12 minutes. The object of this step is to remove any parts of rust that have formed or residues of grease and oily material not removed through the previous degreasing step.

The surface preparation step ends with the step A3, during which grit removal, or dust collection, is carried out, followed by drying and air cooling to bring the small metal parts to ambient temperature (around 21°C).

After the steps of degreasing (Al), sandblasting (A2), and dust collection followed by drying and air cooling (A3), to bring the small metal parts to ambient temperature, the small metal parts are preferably loaded in a perforated container (step A4), then are subjected to the step (B) of application of the anti-corrosion coating.

In this step the cleaned, uncoated small metal parts are immersed in a composition containing a resin selected from epoxy resin, melamine resin, acrylic resin and phenolic resin, a hardener, a catalyst and a lubricant, in an aqueous dispersion. The aqueous dispersion is free of organic solvents. The aqueous dispersion is free of phosphates. The aqueous dispersion does not contain particles or flakes of elemental metals. The concentration of resin in the aqueous dispersion can vary from 75 to 95 wt% of the total dispersion.

The resin is selected from commercial resins capable of forming stable aqueous dispersions. According to an aspect of the invention, the resin is preferably selected from the group consisting of epoxy resins and phenolic resins.

Preferred epoxy resins epoxy resins based on the diglycidyl ether of bisphenol A (DGEBA resins), or epoxy resins based on brominated bisphenol A, or phenol and cresol epoxy novolacs.

Preferred phenolic resins are those obtained from the polycondensation of phenol and formaldehyde. Examples of such resins are novolacs and resols.

The hardener and the catalyst are selected as a function of the type of resin used, as is known to the person skilled in the art. The lubricant is selected so as to be adequate for the resin selected for a given friction coefficient. A preferred lubricant is based on polymers in water. Typically, the lubricant is used in an amount from 5 to 25 wt%% of the total dispersion.

The aqueous dispersion is agitated so as to mix the components well. The immersion time of the uncoated metal parts varies from 10 to 60 seconds, and, if required, rotation cycles of the basket containing the parts are a function of the thickness of the coating to be applied, which varies from 4 to 15 micrometers. In this way, complete deposition of the coating composition on all the small metal parts treated is ensured, so that there are no components that are not wetted by the liquid and, consequently, that are not well coated by the composition. The amount of the composition applied to the small metal parts is a function of the immersion time and of the centrifugation cycle.

After application of the aqueous dispersion of resin and lubricant, the small metal parts are subjected to the centrifugation step (C) in order to remove excess coating present on their surface, and at the same time standardize the surface layer that coats them. The centrifugation time, the rotation speed (from 100 to 600 revolutions/minute) and, if required, the inclination (0-60°C) are a function of the thickness of the coating to be obtained, in conjunction with the effective immersion time. However, this time must be adequate to obtain sufficient surface protection without changing the characteristics of size required for the treated parts. Short immersion and centrifugation cycles produce a deposition of a thin layer of coating, which translates into low corrosion resistance, i.e., a resistance in the salt spray chamber of a few hours. On the contrary, prolonged immersion of the small metal parts in the coating composition produces a thicker coating of the small metal parts prolonging the corrosion resistance time, while complying with the geometrical characteristics required.

The test in the salt spray chamber is carried out according to the standard ISO 9227.

The hardening step (D) of the epoxy, melamine, acrylic or phenolic resin is carried out at a temperature from 110 to 320°C, using an oven (Fig. 4) which can be of the belt, static or tray type. Preferably, the oven is of the fan-assisted type. The curing time can vary, according to type of resin, from 10 to 45 minutes. For this purpose, the small metal parts are preferably unloaded onto a belt to be conveyed through the oven.

The hardening step allows the resin to react with the hardener and with the catalyst in order to obtain a uniform cross-linked polymer, which forms the anti-corrosion coating of the small metal parts and gives them the desired characteristics of resistance to the salt spray test. The temperature and the hardening time are a function both of the type of resin used and of the type of hardener selected.

In the present description the hardening step is also called curing step. The process described makes it possible to obtain an anti-corrosion coating for small metal parts capable of satisfying the salt spray chamber test and, and which is also capable of giving the small metal parts a friction coefficient that is standardized as much as possible. In fact, the coating obtained is surprisingly capable of reducing the variability of the friction coefficient of the small metal parts, improving the performance of the small metal parts particularly in applications for tightening motor vehicle components. Without being bound to any theory, it is believed that this is due to the presence of a polymeric lubricant in the composition. On the contrary, in the phosphating process a lubricating oil is applied on the anti-corrosion coating. This lubricating oil, however, can be worn off during before the actual use of the metal part, for instance a screw, resulting in a worsening of the friction coefficient.

According to another embodiment of the invention, small metal parts coated with the composition according to step (B) can be subjected to a second treatment cycle with the same composition, as shown with the dashed line in Fig. 1. In this second treatment cycle steps (A) are omitted, and the small metal parts delivered from the hardening step are fed directly to step (B). This operation is optional, and is conducted in order to ensure the application of a thicker coating layer of the small metal parts and increase the corrosion resistance characteristics based on the market needs that the small metal parts must satisfy.

This coating operation can be repeated several times so as to ensure a desired level of coating of the small metal parts as a function of the corrosion resistance requirements desired within the limits of the geometrical tolerances requested.

Finally, the small metal parts can be subjected to subsequent optional treatments, in which further finishing coatings can be applied for specific needs, for example to provide aesthetic coloring, to give functional characteristics, such as tracking characteristics, and the like.

EXAMPLES

Below are some examples of the process according to the invention, provided in order to evaluate both the corrosion resistance and the friction coefficient of the small metal parts. Class 8.8 screws and nuts are made of medium carbon steel; quenched and tempered.

In Examples 1-4 hexagon head screws of Class 8.8 M10xl .5x55 were coated with the process according to the invention shown in Fig. 1, and with the equipment shown in Figs. 2-4, in one cycle of treatment. The coating composition comprised a resin selected from the group consisting of epoxy resins, melamine resins, acrylic resins and phenolic resins, a hardener and a catalyst for the selected resin, and a lubricant comprising a polymer dispersed in water. The salt spray corrosion resistance is measured according to the standard test ISO 9227. The samples produced according to the method described above were analyzed also with the method ISO 16047 to evaluate their friction coefficient.

The cleanliness test was carried out according to the standard ISO 16232.

Other process conditions and the results of the test are shown in the tables below.

EXAMPLE 1

EXAMPLE 2

Hexagon head screw Class 8.8 M10xl.5x55 coated with a coating composition comprising a Melamine Resin

Characteristics / Step Unit Value

Degreasing Minutes 10

Sandblasting Minutes 4

Immersion in the aqueous Seconds 20

dispersion of the coating

composition

Centrifugation Revolutions (rpm) 300

Curing time Minutes 25

Curing temperature Degrees centigrade 150

Salt Spray Test Red rust hours ISO 9227 >12

Friction coefficient test 0.12-0.18 ISO 16047 Average 0.16

Cleanliness test ISO 16232 OK EXAMPLE 3

Hexagon head screw Class 8.8 M10xl.5x55 coated with a coating composition comprising a Phenolic Resin

Characteristics / Step Unit Value

Degreasing Minutes 10

Sandblasting Minutes 5

Immersion in the aqueous Seconds 30

dispersion of the coating

composition

Centrifugation Revolutions (rpm) 400

Curing time Minutes 15

Curing temperature Degrees centigrade 190

Salt Spray Test Red rust hours ISO 9227 >24

Friction coefficient test 0.08-0.14 ISO 16047 Average 0.12

Cleanliness test ISO 16232 OK

EXAMPLE 4

Hexagon head screw Class 8.8 M10xl.5x55 coated with a coating composition comprising an Acrylic Resin

Characteristics / Step Unit Value

Degreasing Minutes 10

Sandblasting Minutes 6

Immersion in the aqueous Seconds 20

dispersion of the coating

composition

Centrifugation Revolutions (rpm) 300

Curing time Minutes 20

Curing temperature Degrees centigrade 180

Salt Spray Test Red rust hours ISO 9227 >36

Friction coefficient test 0.08-0.14 ISO 16047 Average 0.12

Cleanliness test ISO 16232 OK EXAMPLE 5 (for comparison)

The same hexagon head screw as used Examples 1-4 was treated with a conventional manganese phosphating process. Both the salt spray test and the cleanliness test gave markedly inferior results to those obtained with Examples 1-3 according to the invention. In particular, the salt spray test gave a result in terms of red rust hours ISO 9227of less than 5 hours, as shown in the table of Example 5 above.

With respect to the friction coefficient test, the values obtained on the metal parts coated with the process of the invention are lower than the values obtained with the phosphating process. Lower values are better since a lower tightening torque, namely less energy, is required when the screws are used, for example, to fasten components of an engine.

Furthermore, metal parts treated with a phosphating process are typically covered with a lubricating oil that is gradually worn off upon handling and transporting the metal parts in the logistic chain from the lubricating step to the actual use. Therefore, the friction coefficient test is not constant but worsens over time. On the contrary, metal parts treated with the process of the invention do not have an external lubricating layer but a lubricant comprised in the composition. They thus exhibit a constant friction coefficient test.

It can be appreciated that the process for the anti-corrosion coating of uncoated small metal parts according to the present invention can replace successfully the conventional phosphating process. It gives better results and does not have the negative impact on the environment of the conventional phosphating process. It can also be appreciated that the process for the anti-corrosion coating of uncoated small metal parts according to the present invention is cheaper than the processes that apply aluminum particles and zinc particles onto the metal parts, but allows forming an effective anti-corrosion coating.