PROTECTION OF GAUGES AGAINST CORROSION

Ing. Peter Stuchlík, CSc., CTex ATI

KORCHEM s.r.o

1. Introduction

All materials are subject to chemical, biological and energetic processes. If the physical, chemical and mechanical properties of the product change during these processes, we speak of corrosion in the case of metals, degradation in the case of plastics, corrosion and degradation in the case of glass and ceramics, etc. The common denominator of these processes in most cases is irreversible changes, which lead to such a change in properties that the object loses its functionality.

Since there are a large number of processes mentioned, this lecture will be significantly limited to length gauges and their construction materials. At the same time, it will also provide practical advice on how to prevent destructive processes.

It is always better and cheaper to prevent problems than to eliminate their consequences.

2. Materials

For length gauges, we can most often encounter the following construction materials:

• metals (Fe alloys, Al alloys, Cu alloys, Zn, Au, Pt alloys, semiconductors),

• polymers (varnishes, paints, PE, PTF, PAD, ABS, PS, PC, PVC, rubber),

• ceramics (porcelain, glass, enamel)

For metals, in terms of their corrosion damage, the most important properties are their conductivity, crystal lattice and electrode potential. In terms of resistance to atmospheric or electrolytic corrosion, the listed metals can be ranked as follows: Fe, Cu, Zn, Al, Au, Pt. Depending on the corrosion environment present, especially the type of electrolyte, some elements may be interchanged. The corrosion of metals is influenced not only by the environment, but also by the alloying elements of the alloys, the contact of two metals of different potential, the contact of two differently treated surfaces, and the shape solution.

The environment from which they were applied (made), their resistance to UV or other high-energy radiation, and their resistance to water and solvents is decisive for polymers. Varnishes and paints are usually applied in the form of solutions, suspensions or emulsions. Some polymers are produced from solution or are produced by emulsion polymerization. In these cases, the environment from which the polymer was applied or produced can damage it. Every polymer is sensitive to high-energy radiation, when the polymer chain breaks, i.e. degradation leading to a change in physical-mechanical properties. In practice, this is most often the effect of UV radiation. This problem is solved with polymers using UV stabilizers, so it is not possible to present an unambiguous range of their resistance. Some polymers hydrolyze in water (e.g. PAD) or swell (e.g. lacquers and paints containing cellulose derivatives). A number of organic solvents attack polymers.

There are also many ceramic materials, but in general they can be said to have a high resistance to chemical degradation processes, while their resistance decreases with the amount of admixture of alkali metal compounds that serve as fluxes. These reduce the resistance, especially in the case of the action of the electrolyte.

3. Types of degradation and corrosion events

For metals, where we talk about corrosion, from a chemical point of view, it is chemical corrosion in an electrically non-conductive environment. That is, the reaction between the metal and the reducing or oxidizing gas at the phase interface. They mostly involve reactions with oxidizing gases such as sulfur dioxide (SO2), sulfur dioxide (SO3), ozone (O3), atomic oxygen (O-), nitrogen oxides (NOx), carbon dioxide (CO3), hydrogen chloride (HCl) , halogens. But the case of hydrogen depolarization using (H+), i.e. corrosion in reducing gases, is quite common.

Furthermore, chemical corrosion occurs in an electrically conductive environment, in the electrolyte, i.e. electrochemical corrosion. I.e. where a conductive cell is formed and the metal(s) form the anode and cathode.

Corrosion caused by processes other than chemical or electrochemical also belongs to this division. This is usually cavitation and corrosion caused by microorganisms.

Due to the fact that for iron alloys, corrosion in an electrically non-conductive environment is minimal up to 580°C, electrochemical corrosion has a decisive influence. Its speeds are up to two orders of magnitude higher than in a non-conductive environment.

According to the method of attack on the material, we distinguish uniform, uneven, point, pitting, lamellar, intercrystalline, transcrystalline and selective corrosion.

According to the phase interface, it is divided into solid phase (metal) with gas, solid phase/liquid, solid phase/solid phase (contact corrosion).

Polymers degrade due to temperature. In thermoplastics, at the glass transition point and simultaneous mechanical stress, deformations and changes take place in amorphous regions without chemical changes. In the region around the melting point, the supramolecular structure of the polymers changes and the mechanical properties are lost. By further increasing the temperature, thermal decomposition is achieved. Thermosets go directly into this decomposition process.

Most polymers were formed by a radical polymerization reaction and are therefore sensitive to radical depolymerization reactions. The initiators of these reactions can be radicals of sulfur oxides, nitrogen, ozone, atomic oxygen, or high-energy radiation, which creates a radical directly in the polymer by breaking the chain. An unpleasant feature of radical reactions is that the free radical has the ability to travel along the chain of the macromolecule to an energetically weaker place, cleave the chain there, releasing another radical that travels on. Therefore, despite the fact that radical reactions at the polymer/gas phase interface take place only in the molecular layer, or by the effect of UV usually only to a depth of 350 nm, by traveling along the chain they are able to disrupt the polymer in the entire volume.

Some microorganisms also cause degradation of polymers. A number of microorganisms produce enzymes that have the ability to split macromolecules into simple substances (usually carbohydrates or acids), which then serve as food for the microorganism. Stopping the enzymatic reaction is very problematic.

Polymers that have an amine, alcohol, or ethoxy group are susceptible to hydrolytic degradation by water. These reactions can be quite fast if an electrolyte is present. For example, PAD degrades easily in an acidic environment. At the same time, in a number of polymers, the residual monomer is washed out, which is then replaced by the degradation of part of the polymer, until equilibrium is established. If this process continues repeatedly, the length of the polymer chain can be reduced to such an extent that the product loses its mechanical properties.

Solvents do not have a direct effect on changing the chemical composition of the polymer, but they can either dissolve it (for example, PS and a number of colors in ketones or acetates) or they can wash out auxiliary substances in the polymer, such as plasticizers, UV stabilizers, etc. In this case, then polymer degradation by mechanical stress or high energy radiation.

Ceramic materials, including glass, are very resistant to mechanical, microbiological and high-energy effects. Their attack occurs due to the content of K, Na, Mg and Ca salts. These salts are either present directly in the raw material itself, for example in porcelain, or are added during production to lower the melting point. They are used as fluxes, especially in the production of glass and ceramics. These salts are decomposed by aqueous solutions of strong acids and bases. As the reaction takes place in an aqueous environment, decomposition products are washed away and corrosion can then continue to the depth.

4. Protection basics

Protection against degradation and corrosion processes begins directly with the manufacturer. If the manufacturer of the device or device neglects something, then the user will hardly be able to correct such a mistake, and if corrosion develops, very often he does not have the means to remove the corrosion without impairing the function or accuracy of the meter. Therefore, it is very important to choose a meter already when purchasing it. Furthermore, the criteria for the correct selection according to the production procedures and materials used will be presented.

The following applies to metals: Basic methods of corrosion protection. They are both physical and chemical. (When we talk about passivation, we are talking about chemical protection, where a chemical reaction occurs between the metal and the anti-corrosion agent. If the chemical reaction does not occur and the agent shifting the reaction balance is attached to the surface of the metal only by physical forces, we are talking about inhibition.)

1. Physical ones work by creating an impermeable layer on the surface that resists the diffusion of electrolytes, oxidizing or reducing substances and has a hydrophobic character.

• plating Cr, Ni, Co, Au, Zn, etc.,

• plastic coating, most often PVC, PP, PE,

• measures with protective coating, varnish, paint,

• by applying a hydrophobizing agent, oil, wax, silicone, fluorinated hydrocarbons, amines, etc.

2. Chemical methods work on the basis of a chemically bound impermeable layer on the surface of the metal, which either transforms the metal into another corrosion-resistant compound, or using a chemical redox reaction prevents the transfer of the corrosion ion to the metal, or acts as a free radical scavenger, or acts as a cathode or anode .

• passivation by oxidation to Fe ₃ O ₄ , blackening,

• passivation with organic salts, oxalate, citrate, tannate, chelate, etc.,

• passivation with inorganic salts, chromating, phosphating,

• inhibition, e.g. amines,

• inhibition using free radical scavengers.

3. Electrochemical methods of protection are based on the creation of a sacrificial anode or the connection of a passive or active cathode.

4. A combination of several principles.

If Fe alloy is used for the bearing part of the measuring device, it is most suitable to be made of stainless steel. However, rust-free stainless steel does not exist, so even this material requires maintenance. If carbon steel is used, it is best to protect it by chrome plating, or if the part is not mechanically stressed, then by PE coating. Conversion layers (such as blackening and phosphating) do not have sufficient resistance in wet environments. Paints and varnishes have limited mechanical resistance and a shorter lifespan against UV.

Furthermore, parts of measuring devices are usually made of Al alloys. The best corrosion resistance in this case is achieved by anodizing.

If a Cu or Zn alloy is used in the meter, and it is not possible to protect it in the factory by gold plating, chrome plating, application of plastic or at least paint, corrosion cannot be prevented. It can only be kept at an acceptable level with certain rules and maintenance procedures.

However, metals are also found in the electrical components of the electronic components of the meters. There are two rules here. Parts exposed to the atmosphere should be gold plated. The electronic part of the given device should have its own dustproof and waterproof case. Dust forms condensation and corrosion nuclei, and electrochemical corrosion is the most progressive.

A high-quality device can also be recognized by the fact that it contains a minimal combination of different metals, especially that there are no combinations of Fe with Cu or Cu with Al. And if it is necessary to use several types of metals, they are non-conductively separated.

An overlooked but nevertheless important element of corrosion protection is the shape solution. Sharp edges increase corrosion attack, while rounded corners reduce it. While notched surfaces hold up well, corrosive chemicals, including sweat, cling to them and moisture condenses well.

With polymers, the situation is somewhat more complicated. Each of them has its own specific characteristics. Therefore, attention will be paid to individual most common polymers separately.

Varnishes and paints.

So-called powder paints have the greatest resistance to chemical influences. This is because they do not use any solvent. Other solution paints are sensitive to organic solvents, especially esters and ketones. From the point of view of water resistance, the most suitable two-component systems are PES or epoxies. The least durable are single-component systems that contain cellulose derivatives as a film-forming component. Disperse paints and varnishes have little resistance to water if they are not cross-linked. PUR and PA have little UV resistance. Inks applied by pad printing are generally soluble in a wide range of organic solvents, including alcohols. Since, with the exception of powder paints, it is impossible to tell what color was used and the manufacturer of the device does not indicate this, it is best if there is no color on the device (if its construction and the materials used allow it). This eliminates the problems of how to protect and renovate the paint or varnish surface.

PE (Polyethylene)

It belongs to the group of polyolefins, but unlike PP, it has excellent UV resistance. In terms of resistance to the effects of water and other chemicals, it is one of the most resistant polymers. It is worse with its resistance to abrasion. A whole range of polyethylenes is produced, which differs not only in the degree of polymerization, but also in structure. Therefore, the basic types are designated: LDPE (low density, branched), HDPE (high density, linear), UHDPE (very high density, linear).

PTF (polytetrafluoroethylene, Teflon)

It has the lowest friction of all known substances, high resistance to water and good resistance to most chemicals. Not resistant to halogenated solvents and UV. It is less resistant to abrasion.

PAD (polyamide)

Due to its good mechanical properties, it is used for gears and sliding bearings. There are several basic types of PAD 6 (Silon), PAD 6.6 (Nylon), aramid (Kevlar). For all types, they have a worse resistance to water, which dissolves residual monomer from them, and to acidic solutions, which can decompose them when heated.

ABS (Acrylobutadiene Styrene)

It is a terpolymer with excellent mechanical properties. It is less resistant to halogenated solvents, esters and ketones. Its UV resistance is also weaker.

PS (polystyrene)

It is cheaper than ABS, so it is used as its cheap replacement. It also has worse mechanical properties, above all it is more brittle. Its resistance to solvents, aging and UV is minimal. If possible, avoid equipment where this polymer is used.

PC (polycarbonate)

Due to its good mechanical properties, it is used for transparent covers. However, it is sensitive to a wide range of chemicals, water detergents and UV.

PVC (polyvinyl chloride)

A polymer with excellent resistance to chemicals, but softens at 40°C, becomes brittle at low temperatures and loses its shape properties around 80°C.

Rubber

Since rubber includes dozens of different polymers, with different fillers and possibly cross-linking, it is impossible to find common characteristics. In any case, care should be taken with organic solvents.

Ceramic materials

There is also a wide range of them, but one can generalize about their excellent resistance to water, chemicals and weathering. Porcelain is attacked by strong acids, glass and glazes by strong hydroxides, few ceramics are resistant to hydrofluoric or fluorosilicic acid.

5. Procedures and means

There is no device that will work forever and that does not require a certain amount of care and maintenance. Next, the most basic principles and procedures will be presented.

To assess the suitability of chemical agents for the treatment and maintenance of devices, a so-called safety data sheet can be used, which by law every agent must have, and which the manufacturer or supplier is obliged to supply upon request. It states which main hazardous substances the product consists of, what the risks are to personnel, the environment, etc. However, not all manufacturers (suppliers) provide complete and true information. Safety data sheets from German manufacturers tend to have the most shortcomings.

The most common enemy of structural materials is water. In addition, with regard to the surrounding conditions, it is in the form of an electrolyte. Water gets on the devices either as rain, or by condensation of atmospheric moisture, such as dew, from detergents, or from fingers.

In the event of rain, the measuring devices should be designed so that water does not get into the interior, where a condensation chamber would form. The surface of the device is dried as soon as possible by wiping with an absorbent material.

Condensed moisture is a bigger problem. The devices should be handled in such a way that there are no sudden changes in temperature, especially if the two environments have different relative humidity. If this cannot be avoided, it is advisable to have a storage cabinet or case for the devices, in which you can put a desiccant (preferably a bag with Silikagel, which, however, needs to be replaced, because it absorbs moisture from the air) and if the device contains materials such as metals , as well as plastics that are sensitive to water, as well as a bag with a vapor corrosion inhibitor (VCI, which lasts up to 10 years). However, it is necessary to consider which preservation system will be used for the given device. In the event that "oil" protective agents are used, the VCI becomes irrelevant because the vapors of the inhibitor through the "oil film" do not reach the surface of the material and sometimes these chemicals can fight with each other.

In general, it is recommended to minimize the use of aqueous detergents. Not only are they electrolytes, but few of them contain anti-corrosion additives, but they usually contain chemicals that are dangerous to a variety of construction materials.

However, every finger touch poses a corrosion or degradation risk. Fingerprints should be removed from the meters as soon as possible. In addition, the composition of each person's sweat is different and changes.

Another common enemy is dust and possibly other pollution. It can act as condensation nuclei, it can have a direct chemical effect, or it can act as a breeding ground for microorganisms.

It follows that it is necessary to clean the gauges after use. Dry absorbent materials (cloths) are not able to remove most dirt and present a risk of abrasion. Therefore, it is advisable to use auxiliary washing liquids. If possible without water, and which have the ability to displace water. Combinations of polar and non-polar solvents are best because they will remove a wide variety of contaminants. Solvents containing halogenated hydrocarbons, ketones and esters are not suitable, as they corrode metals and degrade a range of polymers. The most suitable are agents containing desulfurized alkanes and pure alcohols. As an example, I can mention KORING 792-10, which is a mixture of heptanes and isopropanol. If the device contains polymers sensitive to alcohols (this is usually some rubber or PC), it is necessary to use only alkane agents, even if they have a smaller spectrum of effectiveness. For example KORING 702 (slower drying) or KORING 792-4 (fast drying). These products degrease at the same time. Considering that simultaneously with the removal of surface impurities, preservatives are also washed away, if they were applied to the surface before that, it is necessary to renew the protective coating or lubricant.

To protect metals, it is necessary to decide in advance which of the paths will be used. Metals can be protected with vapor corrosion inhibitors (VCI) or with "oil" preservatives. Combining both methods is not recommended and is sometimes risky. The advantage of most preservatives is that they work on both metals and polymers. Vapor inhibitors are not resistant to liquid water. However, they hardly affect the accuracy (up to nm) and electrical conductivity. The so-called "Oil" preservatives are well resistant to water, but affect accuracy. The protective film of the inhibitor is usually 2-4 m for evaporating, drying "oils", 8-20 m for classic, non-drying "oils", and over 40 m for petroleum jelly. It is also important to note that these corrosion inhibitors are effective dielectrics, so they electrically insulate. Preservative "oils" can also attack some types of rubber, PS and PC. All non-drying "oils" have the unpleasant property of picking up dust and dirt.

From both groups, you can use, for example: vapor corrosion inhibitor for non-ferrous and ferrous metals KORING 505, vapor-water soluble inhibitor KORING 555, washing and preservative drying agent KORING 145-K for non-ferrous and ferrous metals, or preservation oil for non-ferrous and ferrous metals KORING 205 .

If it is a matter of lubrication of non-corroding parts or equipment with a small risk of corrosion, the most appropriate solution is to use silicone oil. It displaces water and creates a strongly repellent surface for a wide range of dirt. The disadvantage is that where it has been used, it will almost no longer stick to anything (it will not have adhesion). The advantage is that it is produced in a wide range of different viscosities, so with low viscosities it can literally be poured into the intercrystalline spaces of a metal or polymer material, so that the moving part is lubricated, but almost nothing remains on the surface. Or you can use high viscosities where it works like petroleum jelly. The viscosity is simply chosen according to need.

In the case of gauges where it has been tested that the agent does not attack the rubber, and where the reduction of electrical conductivity does not play a role, agents of the CLP type (Claening, Lubricating, Protection/Preservation) can be used. These agents clean and wash excellently in one step, at the same time neutralize corrosion reactions, preserve for a long time and provide thixotropic lubrication. One of the best preparations in the world is the CLP Professional from CX Dynamics, originally developed for weapons.

Finally, a few notes on generally used means and procedures.

Very often pure petrochemical products are used for gauges. This has its justification in healthcare products, but it is a mistake with meters. From the very nature of the occurrence of corrosion events, if two differently treated surfaces come into contact in an electrically conductive environment, a galvanic cell is formed. In addition, so-called mineral oils and petroleum jelly are able to bind 4-8% of air moisture, so they are partially electrically conductive. Therefore, it is necessary to use lubricants that contain corrosion inhibitors.

Technical gasoline is a relatively health-hazardous mixture of hydrocarbons, which has very little degreasing and washing power. It is always better to use alkane desulfurized and dearomatized agents.

Kerosene lubricates poorly rather than degreasing. In addition, it is also a rather "dirty" mixture. The fact that some device manufacturers recommend it is an alibi to avoid the risk of polymer damage. There are suitable remedies for everything, so there is no reason to use kerosene.

The vast majority of ethanols (commonly known as alcohol) are not pure, they are already contaminated during production and denatured. Only ethanol marked pa and partially even pharmaceutical grade can be considered pure. However, the washing ability of ethanol is much lower than that of isopropanol. Therefore, it is better to use isopropanol or special products that contain it.

Preservatives with the longest possible shelf life are always chosen. If, for some reason, it is necessary to use a product with a short protection period, or the protective effect is canceled by an external influence (washing, etc.), it is necessary to modify the instructions for using the device so that it corresponds to the given case.

The transmission mechanisms of the gauges do not need to be lubricated if they are made of some sliding polymer, e.g. PAD. In other cases it is necessary. Either a suitable silicone oil, or an evaporative drying preservative, or a water-soluble VCI.

If washing or de-preserving is carried out, it is desirable to carry out preservative treatment as soon as possible. In particular, corrosion processes in a conductive environment are very fast, taking seconds.