Zinc Coating by Electrogalvanizing Method

1. Effect of Galvanizing on the Base Material

Constructions and components are typically subject to stresses that strain the mechanical properties of the base material. This is equally true for components that undergo electroplating to provide corrosion protection. For this reason, galvanizing can affect the mechanical properties of the base material. According to various studies, the effects of zinc coatings produced by electroplating on tensile strength such as tensile load are minimal and less than the effects of other metals that accumulate as a result of the electroplating process (with the exception of chromium, which prevents deformation). Accordingly, even the internal stresses in the main zinc layer can be said to have no effect on the base material. In galvanizing carried out in acidic electrolytes, no effect is observed on dynamic fatigue properties such as fatigue strength and structural strength. However, this is not the case when galvanizing in alkaline electrolytes with low current efficiency and high hydrogen evolution. As a result, hydrogen charging occurs that may be stronger or weaker depending on the properties of the base material and can cause hydrogen embrittlement. For example, in bending tests conducted with galvanized spring steel samples, it was found that when the zinc layer was separated from an alkaline electrolyte, the base material exhibited significantly poorer behavior. The bending coefficient and increasing zinc layer thickness with the number of bends to fracture change with difficulty when the layers are in acidic electrolyte, but drop sharply when alkaline electrolyte is used (see Figure 1). In addition, many investigations have shown that the primary effect of galvanizing stems from hydrogen charging and the resulting hydrogen embrittlement. The cause of these brittle fractures in the material is generally hydrogen absorption. Since this generally occurs in cracks without a change in shape at the macro scale, it can be very dangerous for the behavior of materials and components. They do not appear immediately; the material is used for a long time without being noticed beforehand and then suddenly appears. For these reasons, the terms hydrogen-induced crack formation or cathodic stress corrosion cracking are used in place of hydrogen embrittlement, particularly in scientific literature. This problem has become more chronic in recent times, particularly with low-alloy or unalloyed high-strength steels, which are increasingly subjected to higher levels of heat treatment to gain more benefit from the steel's strength. The material loses its toughness in this way and becomes particularly sensitive to hydrogen diffusion.

2. Hydrogen Embrittlement as a System Property

Embrittlement of a material is defined as a state of low ductility in which the absorption of relatively small mechanical energy can cause irreparable damage. If this occurs due to hydrogen entering the material, it is called hydrogen embrittlement. Under certain conditions, as a result of hydrogen absorption, the elastic properties of the base material are greatly affected, it becomes brittle and does not show sufficient resistance to gradually acting mechanical loads. Although behavior against sudden strain such as impact or stress changes slightly, a strong effect is observed in the plastic range. Properties such as tensile strength, notch impact strength and the like are generally not affected, but ductility, yield strength, elongation, contraction, bending strength etc. are affected. As mentioned previously, this result can be described as brittle fracture, representing very dangerous and usually sudden material disintegration. The hydrogen effect on the base material or component depends on three conditions: sufficient hydrogen must be present, the base material must undergo sufficient hydrogen charging, and finally, appropriate conditions must exist that enable the mechanical properties of the material or component to be affected by hydrogen. All factors and parameters can influence each other positively and negatively. Therefore, hydrogen embrittlement is usually dependent not on just one, but on multiple influencing factors and can be attributed to various causes. For this reason, it has the character of a system property.

2.1 Hydrogen Supply in Electroplating

In galvanic surface treatment, hydrogen can enter the material or base material in three process steps: • During pickling, • In cathodic pre-treatment, and • In electrolytic metal deposition. It can be clearly acknowledged that the dominant part of hydrogen entering the material comes not from electrical deposition, but from pre-treatment. According to common opinion, hydrogen embrittlement originating from zinc deposition (and also from other galvanic coated metals) is incorrect in most cases. Pickling is an electrochemical reaction in which oxidation of the material occurs in anodic regions, resulting in reduction of hydrogen ions and formation of hydrogen atoms in cathodic regions. This process also occurs on cathode surfaces due to electrolytic degreasing and electrical metal deposition, but only when current efficiency is less than 100%. Hydrogen atoms are first adsorbed on the component surface and then (depending on conditions, faster or slower) recombine into hydrogen molecules that exit as hydrogen gas. These processes can be explained as follows: Hydrogen ions (protons H+) are generally bound to water and form in the form of hydronium ions (H3O+). Hydronium ions are released from the hydrate shell in the material/liquid at the phase boundary, discharged through reduction (electron acceptance), and bound to the metal surface: H3O+ → H2O + H+ H+ + e- → Had Both bound hydrogen atoms can recombine on the material surface to form a hydrogen molecule: Had + Had → H2,ad Bound hydrogen H2,ad diffuses away into solution or exits as hydrogen gas. Hydrogen thus passes to the metal surface in both atomic and molecular form.

2.2 Hydrogen Diffusion Mechanism in Galvanizing

For hydrogen to diffuse from the material surface, it must be in atomic form. Hydrogen atoms penetrate the material due to their small diameter, filling lattice defects, grain boundaries and micro-crack areas, and can also pass through the interior of crystals (depending on conditions present). Surface layers are formed that can reach a depth of 30 μm affected by hydrogen. Molecular hydrogen cannot penetrate through diffusion. If the surface is highly porous or if there are larger cracks or voids, hydrogen can penetrate even in molecular form. However, at high hydrogen concentrations at phase boundaries, hydrogen molecules can be seen to dissociate again and diffuse in atomic form. Apart from being in the appropriate form, hydrogen must also be in direct contact with the surface of the base material. For diffusion to occur, it must be partially filled with hydrogen. However, in electrolytic galvanizing, the absorbable hydrogen at the phase boundary is distributed between the base material and the zinc layer. However, the properties of the base material can only affect the rate of diffusion at the surface. How large this rate is depends on both the electrolyte and galvanizing conditions and the base material.

2.2.1 Effect of Electrolysis Conditions

During the galvanizing process, hydrogen is deposited simultaneously with zinc, starting from certain active regions, and then forms a dense zinc layer over the entire surface. Therefore, adsorption of hydrogen atoms occurs first at the phase boundary between base material and electrolyte, then at increasing rates at the phase boundary between zinc layer and electrolyte. The more zinc is deposited, the more the hydrogen overloading rate increases. Finally, coating the entire surface with a dense zinc layer prevents hydrogen atoms from diffusing further into the base material. Generally, a layer about 0.5 to 1 μm thick forms in this case. A 2 μm thick layer constitutes a good diffusion barrier against hydrogen. The time required for such a layer to form is critical for hydrogen diffusion into the base material and thus for hydrogen embrittlement. The length differs for different zinc electrolytes and is as follows: • For high-cyanide electrolyte 56 mAs/cm², • For alkaline cyanide-free electrolyte 18 mAs/cm², and • For weakly acidic electrolyte 6.7 mAs/cm². After this layer forms, hydrogen diffusion does not occur in the base material. The current efficiency of the electrolyte has no effect on hydrogen loading. Hydrogen is only absorbed in the zinc layer. Two mechanisms stand out due to different hydrogen absorption. While the pore mechanism generally applies to Canadian baths without brighteners, the absorption-diffusion mechanism applies to baths containing brighteners. These mechanisms are distinguished from each other according to the different source of diffused hydrogen. On the other hand, in the absorption-diffusion mechanism, hydrogen absorbed in the zinc layer and penetrating from there into the base material is involved, while in the pore mechanism, hydrogen absorbed at the beginning of deposition at open points in the base material is involved. The second process is slower because less hydrogen is absorbed in the zinc layer. The lower the hydrogen concentration gradient, the less the zinc layer should preferably contain hydrogen compared to before. Therefore, the rate of hydrogen absorption in steel and changes during deposition are mainly dependent on layer formation parameters and are less visible in coated steels. As a rule, as long as zinc deposition is faster and forms a dense layer, the hydrogen loading risk decreases proportionally. For this reason, operation at high current densities is suitable.

2.2.2 Effect of Base Material

Emphasis has been placed on the tendency of unalloyed or weakly alloyed high-strength steels to absorb hydrogen. Apart from this, steels with carbon content >0.5% should be more sensitive to hydrogen charging. Opinions on the effect of chromium, tungsten and nickel alloy components vary. Micro-cracks, micro-pores, voids and other irregularities on the surface increase hydrogen penetration. These defects can occur if the material is severely deformed before galvanic coating or becomes rough or damaged by mechanical grinding or polishing. A chemically or electrochemically polished surface absorbs only 30 to 40% of the hydrogen compared to a mechanically processed surface. However, rough surfaces can also reduce the hydrogen charging tendency when hydrogen atoms in rough areas preferentially recombine as molecules. Hydrogen diffusion is preferred when surface contaminants, contact poisons such as carbon, phosphorus, lead or tin are present. These reduce hydrogen overpotential and slow the growth of the zinc layer. The structure has a significant effect. Hydrogen charging in the base material is particularly high under strain or deformation. This can be prevented by prior cold working or heat treatment (hardening, carburizing). In these cases, hydrogen enters the lattice and, if it subsequently exits at room temperature or higher temperatures, causes lattice shrinkage. Theories regarding hydrogen behavior in microstructural anomalies are quite diverse.

3. Prevention of Hydrogen Embrittlement

Hydrogen embrittlement is a system property. If it is to be prevented, measures must be taken in all areas affecting both the material and component and galvanic surface treatment. Generally, measures based on the material and component should be preferred. Critical hydrogen charging can in most cases only be prevented after the last process in the production technology chain, which means that surface treatment takes place and the defect is therefore found later. In such critical cases, measures as possible in the galvanizing department of surface treatment must be taken. In practice, hardened parts, tempered screws, springs and parts made from high-strength steel or parts produced by cold cutting, cold rolling or drawing, hydrogen embrittlement should be expected. In technically important unalloyed carbon steels and low-alloy tempered steels, tensile strength of 1200 N/mm² should be considered critical; to prevent this, the measures described below must be taken. Materials made from titanium are also among materials sensitive to hydrogen embrittlement. The efficiency of such measures can be determined according to DIN 50969 or by an approved test.

3.1 Measures for Galvanic Surface Treatment

Measures in galvanic surface treatment are possible or necessary in pre-treatment, electrical deposition and post-treatment interventions.

3.1.1 Pre-Treatment Measures

• Exposure times for all pre-treatment procedures should be kept as short as possible. • Pickling with the addition of inhibitors that do not simultaneously prevent recombination of hydrogen atoms into H2 molecules. Especially when sensitive parts are involved, the pickling process should be carried out in a protective gas atmosphere. If hydrogen charging during pickling is unavoidable, an intermediate tempering process may be necessary (usually 0.5 hours at 150°C). • For high-strength parts, particularly for safety parts, only mechanical methods such as grit blasting (blasting with defined aluminum oxide as the blasting material in cases of risk, particularly in aviation for parts at risk) should be used for cleaning and activation. • Electrolytic degreasing only by anodic absorption.

3.1.2 Measures for Main Galvanizing

• Selection of a well-covered electrolyte to quickly reach the minimum barrier layer thickness of 0.5 μm that prevents hydrogen diffusion. As a rule, acidic electrolyte is suitable. • Under conditions achievable in a specific application, use of electrolyte with 100% current efficiency if possible. • Carrying out the electroplating process with properly changing direct current. By selecting appropriate parameters, crystallization conditions can be provided that allow fine-grained and low-stress layers to be obtained.

3.1.3 Post-Treatment by Tempering

• Regardless of which process step is carried out, post-treatment tempering is done no later than 24 hours later. Most of the hydrogen is in the first zinc layer 0.1-0.2 μm thick. For this reason, if annealing is performed immediately after the galvanizing process, the hydrogen enriched at the phase boundary may diffuse out of the zinc layer rather than into the base material. However, this should also not be too thick and should not come from a brightening bath. If the time until tempering is extended, not only hydrogen from the phase boundary but also, in some cases, hydrogen from the bright zinc coating can diffuse into the base material. • To prevent damage to the base material and zinc layer due to hydrogen spreading very rapidly, heating to the desired temperature should be carried out gradually. • If there is a possibility of hydrogen (from a thin, additive-free zinc layer) escaping, holding in an oil bath at 140 to 200°C for 2 to 3 hours is generally sufficient. For components with higher degrees of deformation, longer holding times may be necessary. • In high-strength steels and high-stress structures, such as in hardening, the tempering temperature and maximum temperature limited by material condition and tempering time should not be exceeded. Guide values are given in Table 1. * Heat treatment should be performed immediately after the galvanizing process.   İzzet Aydın General Manager Hillebrand Chemicals Kimyasal Pazarlama Ltd. Şti.     Sources: Metalloberfläche, Paatsch W. DIN 50969. Metalloberfläche, Tolls E. Materials Performance, Isecke B. Zasc. met., Schitanow W.J. Korr. und Metallschutz, Fischer H.