Keys to selecting a good coating applicator

Article courtesy of Atotech (www.atotech.com)

Logo Atotech Inelca Consulting

Authors:

Steve Kocka: General Manager General Metal Finishing North America at Atotech USA LLC

Markus Ahr: Global Product Manager Corrosion Protection Coatings, Atotech

 

Selecting the right electroplating applicator can be as challenging as producing the part. These are the key considerations to ensure a quality finish and reasonable delivery.

As the industrial world continues to evolve, the need for metal surface coatings also changes. The challenges posed by the end owner of a given part become more complex. There are several points to consider when choosing the appropriate finish applicator.

Just as there are different levels of quality among metal fabrication and metalworking facilities, there are also many different levels among applicators in the coating world. Choosing the right coater is an important step that is often overlooked in the final stage of manufacturing. To get the job done right, there are a few basic points to consider:

  • Logistics: Logistics can play an important role. In some cases, the location of the finishing supplier is critical as transportation influences the final cost of the finished part. Some applicators have their own fleet of trucks.
  • Organic versus inorganic: Requiring organic finishes (paint) or inorganic finishes can have an impact on the choice of the most suitable applicator for the job. The selection of a coater that meets the quality requirements as well as the final OEM specification is an important consideration. The size of the parts to be processed can also be a limiting factor. Does the part fit in the paint booth / tank / cataphoresis tank…? Some OEMs specify very specific process chemistry, and not all applicators will have those finishes or chemistries. Thoroughly examine the capabilities of the coater before committing to the job.
  • Coatings: These days, a single layer of coating is often not enough to meet the end user’s decorative or corrosion criteria. This is where things get more complicated. Finding an applicator that can provide both organic and inorganic finishes is a key element in this area. As an example, the finish can be an electrolytic zinc or zinc/nickel coating, followed by a RoHs-compliant trivalent passivate, an inorganic finish such as cataphoresis, powder coating, wet painting, adhesives (rubber bonding) or zinc flakes, all of these possible combinations of coatings offer completely different aspects and properties to what a single layer coating can offer. Working with an applicator whose chemical supplier has all the necessary finishes will prevent us from being held responsible if coating problems arise.
  • Thickness: The final coating thickness specification is also a critical element when choosing an applicator. To apply dip/spin zinc flake coatings, appropriate equipment is needed to minimise build-up on the head or thread (for fasteners) or to avoid blocking threaded blind holes. Not all applicators have such application equipment available.
  • Quality: Does the applicator have a quality system? Accreditations such as CQI (GM) or DIN EN ISO 9001, IATF 16949, etc. can provide the necessary level of quality. Not all applicators have advanced quality systems in place.
  • Part assembly: If a part is to be assembled with another part from a different origin or process, decorative concerns such as “colour matching” or functional concerns such as “galvanic corrosion concerns” may arise. A typical example in the automotive industry is assembling aluminium structures using coated steel screws.
  • Testing: The applicator should have access to a technical centre where he/she can apply and compare various types of coatings for evaluation, as well as be able to perform the required corrosion and coefficient of friction tests.
  • Equipment: does the applicator have all the necessary equipment or will he need to incorporate new special equipment? Some applicators make their own accessories, which can optimise production times and add value to the supply chain by completing more steps in one place.
  • Coating supplier: In some cases, the company that supplies the chemistry and materials for the coating to the applicator may have significant resources at its disposal:
    • In-house technical centres with the capacity to process large parts, thus being able to provide real production samples with a variety of coating processes.
    • Analytical and metallurgical laboratories that can examine surface conditions as well as analyse processes, assisting in both defining a process and identifying the causes of potential defects.
    • Multiple coating capabilities, which can offer the OEM or Tier the opportunity to explore all types of coatings or, in some cases, coatings that combine organic and inorganic materials.
    • A catalogue of already proven single and multiple coating finishes, with their known properties, which will provide preview results and assist in faster design of the final finish.
    • Global services, which allow “copy-paste” of the desired finishing systems already being applied in other parts of the world, thus obtaining uniform results in the supply chain.
    • Global audit programme, which guarantees a standardised quality system for the coating industry, which will be well received by Tiers and OEMs.
    • Customised training courses on the possibilities and limitations of coating systems for OEMs and TIERs to develop the ability to distinguish and evaluate different levels of quality.

When faced with the challenge of defining a coating type as well as a network of applicators that meets the needs of the production process, involving the coating chemistry manufacturer in the process can often shorten the market entry cycle.

Evaporation: Environmental solution

Imagen manos y agua para Evaporación: Solución medioambiental

As part of Inelca’s continuous effort to minimise the environmental impact of its activity, our company has incorporated a mechanical compression evaporator for the treatment of waste water from the Zinc Nickel (ZnNi) processes.

The incorporation of this water treatment technology is intended to cover several objectives:

  • Minimisation of water consumption: By means of the mechanical compression evaporation technique, water consumption is reduced by up to 97 %.
  • Reduction of environmental impact: By reusing up to 97 % of the water treated in the evaporator, the environmental impact of this volume is reduced, approaching zero discharge.
  • Robustness: The evaporation processes make the purification process robust and reliable.
  • Water quality: The water obtained after the evaporation process improves the quality of the mains water, giving greater stability to the production processes.

After one year of operation, the conclusions are positive and all the established objectives have been met. Despite being a considerable investment, the results obtained endorse and justify it.

Post-pandemic economic situation in the electroplating sector

After more than a year of trying to overcome the serious problems caused by the pandemic, and when it seems that the clouds are beginning to clear with the immunisation of the population through the vaccine, new obstacles are appearing on the horizon in the industrial sector that only hinder the exit of companies.

Focusing on the automotive sector, one of the main sectors of the electroplating industry, more than 231,000 vehicles have not been produced in Spanish factories so far this year. One of the main reasons for this is the worldwide shortage of semiconductors, and it seems that a solution to the problem is not in sight until 2022.

To this problem must be added the huge increase in the price of raw materials such as metals, chemicals and oil, which is not only affecting the cost of plastic materials, but also energy and logistics costs, as in the case of sea freight, the cost of which has increased fivefold. The high electricity prices we are seeing and the regulatory changes in the electricity bill that came into force this June are not helping companies either, further damaging their bottom line.

There have been several reasons for these increases: the increase in demand for raw materials due to the gradual recovery of the markets after Covid-19, the tendency of companies to over-stock to avoid stock-outs, the reactivation of the Chinese and US economies, which has led to a lower flow of materials to Europe, and the 35% drop in Europe’s steel production capacity.

For all these reasons, the Spanish automotive supplier industry is now facing a crisis derived from the lack of raw materials and rising costs, which is holding back the path of recovery it was starting on.

There is now talk of higher than desired inflation, but anyone who knows the automotive sector knows how difficult it is to raise any price increase derived from cost increases with the manufacturer, especially when it is at the end of the production chain as a treatment supplier, so it is they who are bearing these increases at the expense of their profitability, putting the viability of the projects at risk.

Electrodeposition

Electrodeposición imagen abeja INELCA

Electrodeposition is the phase of the electrolytic treatment where the parts are coated with a thin layer (microns) of the metal or alloy that we want to deposit.

To do this, the parts are immersed in an electrolytic solution, called electrolyte, which contains ions of the metal or metals that will form the layer.

A galvanic system is created in which we have an anode, a cathode (the parts to be coated are those that act as a cathode), the electrolyte, where the ions of the metal to be deposited are (Zn, Fe, Ni, …), and a source of direct current that provides electrons, by means of which the reaction of reduction of the metal ions is produced, to be transformed into metal, on the surface of the parts to be coated.

A simple diagram of this galvanic system would be as follows:

Electrodeposición esquema INELCA SLU

The main factors that influence the correct formation of the coating layer are:

  • State of the surface to be coated.
  • Composition of the electrolyte.
  • Conductivity of the system.
  • Applied current density.
  • Working temperature of the electrolyte.
  • Agitation and filtration of the electrolyte.

All these parameters are controlled at INELCA according to our control plans.

Prevention & Innovation

The spread of the Covid-19 pandemic has forced us to quickly alter our habits. It is now up to companies to ensure compliance with rules of conduct that allow safe activity. That is why INELCA, aware of the culture of prevention and complying with the Law on the Prevention of Occupational Risks, has incorporated the procedure for action and contingency plan for the Covid-19.

We believe and understand that the multiple measures and actions applied for the safety of the employees, are not temporary measures but a structural change that comes to stay for a long time. And it is in this new framework where innovation can help us and where we have chosen to install temperature detection and access control solutions for early prevention as containment measures. Solutions with high thermographic reliability together with functionalities provided by artificial intelligence providing security, precision and speed. All aimed at a clear and unique objective: the safety of all our employees.

Preparation/Cleaning

For a correct application of any surface treatment, the state of the surface on which it is to be applied is essential.

Because the parts on which the coating is to be applied come from previous processes (stamping, machining, threading, heat treatment, etc…), they must be subjected to cleaning processes prior to the deposition of the coating, otherwise we may find problems of adhesion or even non-coating. This is what is called the preparation phase within the coating processes.

Within the coatings that we apply at INELCA, we can differentiate between two large groups, electrolytic (Zn,ZnNi, ZnFe) and non-electrolytic (Zn flakes), and each of them has a different type of preparation:

Electrolytic:

The preparation phase of electrolytic coatings consists mainly of three stages: chemical degreasing, electrolytic degreasing and pickling.

i)Chemical degreasing: This is an alkaline phase, normally based on soda, responsible for eliminating the oils from the mechanisation of the parts or from the threading processes.

ii)Electrolytic degreasing: This is an alkaline phase, normally based on soda, in which the cleaning effect of the chemical degreasing is added to the mechanical action due to the generation of gas on the surface of the part. This gas is generated by a chemical reaction when an electric current is applied. Thanks to this mechanical action, it is possible to clean up oil residues occluded in the pores of the parts.

iii)Pickling: This is an acidic phase in which metal remains and oxides present in the parts are eliminated, both those that come from natural oxidation and those that may be produced in heat treatments.

In this phase the intermediate coatings for storage that the pieces can have are also eliminated.
The most common acids used in this phase are hydrochloric or sulphuric, to which inhibitors are added to prevent over-attacking of the parts and to minimise possible hydrogen embrittlement.

Zn flakes:

Zn flakes processes, to avoid hydrogen embrittlement processes, have a different preparation. The most common consists of chemical degreasing and shot blasting.

i)Shot blasting: Mechanical cleaning process that consists of projecting abrasive material onto the parts.

Corrosion tests II: Most common types of tests

Imagen para enseyosde corrosión INELCA SLU

In the previous instalment of this publication we saw the definition, purpose and limitations of corrosion tests, in this instalment we will discuss the most widespread types of corrosion tests used to test metal parts coated with sacrificial metals such as Zn, ZnFe, ZnNi and Zn flakes.

SST:

The initials of this test correspond to the Salt Spray Test, currently it is perhaps the most extensive test to evaluate the quality of coatings deposited on steel parts. It consists of introducing the pieces to be tested in a chamber that is filled with mist by means of a nebulizer formed from a solution of distilled water with 5% NaCl (salt), with humidity, temperature and pH of the chamber under standardized parameters. The permanence within the SST will depend on the type of coating to be tested and the specification established by the end user of the parts. During the test, which can go from 24 h to 3000 h or more, the condition of the samples is evaluated every 24 h, and both the corrosion of the coating (WR: white rust) which will appear first, and the corrosion of the steel (RR: Red rust)

Climatic tests:

As we discussed in the previous instalment, corrosion tests cannot be correlated with the useful life of the piece-coating assembly, in order to bring the time of use and test relationship closer several years ago the automotive sector began to specify climatic tests on treated parts. For this, each manufacturer defined a unique test cycle tailored to their needs, thus creating another type of corrosion test: Climate Tests. These tests consist of cycles of usually 24 hours (1 cycle) consisting of subjecting the samples to a sequence of climatic conditions of humidity, SST and sudden temperature changes. The permanence of the samples in the climatic test chambers will depend on the applied coating, the specification of the end user and the type of test defined by each OEM, ranging from 1 to 30 cycles or more.

Covid-19 tests

The Inelca staff underwent tests last week, at its facilities in Sant Esteve de Sesrovires, to check if they are affected by Covid-19 and thus ensure the health of each of them. The results have been satisfactory, allowing the activity to continue safely.

In addition, the facilities have been adapted to the health protocols required by the authorities to minimize the risk of contagion and create a safe work space. These protocols require an effort by everyone, but the general attitude towards this new situation is collaborative and positive.

With all these actions carried out, the level and quality of the response is maintained to provide service to our customers who demand metal surface treatments.

ZnNi and Hydrogenation

One of the drawbacks of electrolytically deposited coatings is the possibility of hydrogenation.

In some of the phases of the coating application process, such as pickling or the deposition phase itself, hydrogen formation occurs on the metal surface to be coated as an auxiliary reaction.
This hydrogen, in monatomic form, can diffuse within the metal structure, producing fragility in it.

This monoatomic hydrogen diffused in the structure of the metal to be coated must be extracted, by means of thermal treatments, before it changes to diatomic hydrogen, at which time it is no longer possible to extract it by conventional dehydrogenation methods.

Zinc-nickel, due to the properties of the deposited layer and the deposition characteristics of that layer, has a much less tendency to hydrogenation than other Zn coatings and other alloys.

i) Properties of the deposited layer:
The deposited zinc layer is not very porous, preventing the hydrogen, which may have diffused in the metal structure to be coated, from being expelled if not by means of heat treatments. On the contrary, the zinc-nickel layer is much more porous and allows the possible diffused hydrogen to be evacuated before going to diatomic form.

ii) Deposition characteristics:
In the first moments of the deposition phase of the zinc-nickel coating, a small nickel layer is generated on the surface of the metal to be coated. This nickel acts as a catalyst in the reaction of the passage of monoatomic to diatomic hydrogen, so that, in this way, it does not diffuse within the base metal structure.

Advantages of ZnNi

Among all the surface coatings of zinc and its alloys, zinc-nickel has a number of advantages that makes it the leader:

i) Greater resistance to corrosion: The presence of 12 to 16% Ni in its structure, allows it to acquire the gamma phase, which gives it greater resistance to corrosion in salt spray and in climatic cycles compared to other applications from Zn.

ii) High resistance to wear: The hardness of the coatings prevents them from being damaged in the coating process itself, as well as in subsequent manipulations such as packaging, selections, transport, assembly, etc.

Layer hardness of various coatings:

Zn: 100 HV
ZnFe: 150-200 HV
ZnNi: 500-550 HV

iii) Reduced white corrosion formation: White corrosion in the ZnNi coating appears much less bulky than in other Zn coatings

Imagen corosión recubrimientos niquel

iv) Good stability at high temperatures: The ZnNi deposit is very stable at temperature, even keeping its properties at working temperature up to 200 ºC.

v) Low Hydrogen embrittlement: Due to the characteristics of the layer of the deposit formed, as well as those of the deposition process, ZnNi presents low Hydrogen embrittlement, as already indicated in standards with ISO 4042.

vi) Low corrosion by contact with Aluminum: Due to the difference in oxidation potentials of Zn and ZnNi, the latter has much less galvanic corrosion by contact with Al, which makes it a coating widely used in both the automotive and in other sectors.