Washed and rusted

Question: We have a lot of parts electroplated by a neighboring electroplating shop. Occasionally, bright steel parts also have to be washed, which are stored and processed by us as required. It often happens that these parts rust after a very short time, while other parts survive the storage period without damage. In the past, the electroplating department washed the parts again and produced an 8D report. At some point, the reports stopped, and now the electroplating shop even charges us for the rework. We, on the other hand, refuse to pay for the rework and the situation threatens to escalate. Our position is that the electroplating company is to blame, as some deliveries are OK and others are not. Who is right?

Answer: Even if the electroplating company's approach seems radical and we would have preferred a more diplomatic approach, we can understand the situation. What you describe is not an isolated case, but a dispute that has been going on for decades in this and similar forms. This not only affects bright, washed steel parts, but also similar surface treatments or all those that have no or little protection or too little for the respective requirements. Your case is particularly extreme because a highly reactive surface is created which you hope will not react. There is a good reason why the supplier eventually stops responding to complaints and claims: to the electroplating department, it sounds as if you keep asking why ice melts faster on warm days than on cold ones.

To make the problem understandable, we will try to recreate the process and explain what happens on the surface. At the end we will list some possible solutions.

Work sequence

As the initial condition of the steel parts can vary, electroplating requires a workflow that covers all the usual eventualities and leads to the same result. Such a process usually looks like this:

1. Degreasing in a hot cleaner
2. Pickling in hydrochloric or sulphuric acid
3. Treatment in an electrolytic degreaser

To 1: The coarsest dirt, i.e. grease, oil and particles, is removed here. This often involves a very alkaline, warm solution. Nowadays, more and more cold, less alkaline cleaning solutions are also being used. If there are no corrosion products on the surface, the treatment is already complete.

Re 2: Removal of rust or oxides. The treatment normally only takes 3-5 minutes. If the steel parts are heavily rusted, the treatment can take considerably longer. Rust scars cannot be removed here.

Re 3: With 2. a coating called "pickling bast" can form. This is removed in the final degreasing stage. In addition, further soiling is removed and the surface is alkalized to prevent immediate corrosion caused by acid residues.

Naturally, extensive rinsing is carried out between each step. After electrolytic degreasing, an attempt is made to rinse a little less so that a slightly alkaline film remains on the surface without stains forming or salts/hydroxides crystallizing out. Finally, the parts are hot-dried and packaged according to the customer's specifications.

Risks

As the process described has been tried and tested for around 100 years and is also carried out in this or a similar way as a pre-treatment before electroplating, the risks at this point are minimal. Provided there are no problems with the part geometry, the only risk is if the treatment time in the pickling bath is too long. We are not talking about minutes, but hours, during which the parts could be attacked by the comparatively strong acid. As more modern pickling processes have so-called pickling inhibitors, the risk here is also rather low.

The final rinse can be a sticking point. If the water is not at least neutral, but slightly acidic, premature corrosion may actually occur. As a rule, the final rinse stage consists of recirculated water, which is monitored by sensors. Despite all caution, problems can also occur here, even if we classify them as quite minor. A problem can arise if the sink is used for pickling and degreasing at the same time and the water was acidified before the last rinsing step.

Depending on the design of the parts, acid residues can accumulate in gaps and cracks that were not sufficiently removed by the last degreasing stage. The electroplater deals with this problem by either degreasing electrolytically for longer or by working with a so-called dewatering solution after the last rinsing step, which displaces the water or acid residues from the gaps.

The real crux of the matter is drying. Depending on the part geometry and therefore the type of processing (drum, rack), different concepts are used and the bare steel is exposed to the incoming air without protection. Barrel parts are normally dried in centrifuges, which is very efficient due to the combination with hot air. For rack parts, the risk of corrosion during drying is higher, which is compensated for by higher temperatures and blowing speeds where possible.

Fig. 1: Diagram of rust formation on steel under a drop of water - graphic: Private

Transportation, storage and condensation

Everything that happens after drying poses a significantly higher risk. This can occur immediately after drying or during transportation and subsequent storage. In our experience, more than 90 percent of the difficulties you describe come from this area. The key word is "condensation" or its formation.

There are the following possibilities for the formation of a film of water on metal surfaces:

Condensation on cold surfaces in a warm environment. A film of water formed in this way is called condensation water or dew. This problem occurs, for example, in the area of transportation/storage

Preferential condensation on condensation nuclei. Dust, pigments and soot as well as other solid impurities act as such nuclei. Depending on the amount of vapor, droplets or thin layers of liquid form.

The formation of thin, invisible water films occurs through the adsorption of water molecules and is not always recognized. For example, a steel surface still appears "dry" even if there are already 2 g of water per^m2 on it. At 20 g of water per^m2 it is still described as "almost dry" and only at around 200 g of water per^m2 does the surface appear "dewy".

For the electroplating customer, storage is often optimal if he does not "feel" any moisture on the surface. Depending on the day of delivery (temperature and humidity), the conditions vary between optimal and catastrophic. When the room temperature cools from 22 °C to 16 °C, the relative humidity rises from 70 % to 100 %. Complete condensation occurs under these conditions.

The problem starts as soon as the truck is loaded in the electroplating shop. The goods may appear to be dry, but electroplating shops are usually warm places with relatively high humidity. During transportation, the goods cool down, the water condenses on the surface of the goods and arrives at the customer wet. This effect is also known from bathrooms, especially in winter. Water droplets form first on the window pane, usually when the heating is on. This is where there is the greatest temperature difference in the room.

In neutral water and humid air, a layer of rust forms on unalloyed and low-alloy steel. Rust is a collective term for the non-uniformly composed oxides and oxide hydrates of the predominantly trivalent iron, whereby the main component is usually formed from FeO(OH) - _nH2O. As an anodic reaction, iron dissolves as a divalent cation, while hydroxide ions are formed cathodically at the same time. In water, precipitates of Fe(OH₎₂ are formed when the solubility product is exceeded. In the presence of dissolved oxygen, the divalent hydroxide oxidizes to form the trivalent, red-brown coloured iron oxide hydrate. This forms a voluminous, loose rust layer on the steel. Figure 1 shows rusting under a drop of water. In this case, the anodic dissolution takes place in the center and rust forms at the edges. In principle, this rusting corresponds to an aeration element, as the _O2 ingress takes place at the edges.

If the surface were uniformly wetted in the atmosphere or under water, the corrosion would proceed uniformly. With uniform rusting, the formation of the rust layer would inhibit the supply of oxygen through the layer, the corrosion rate is then reduced.

In the marine atmosphere, corrosion is particularly favored by the chloride content. The relative humidity in this atmosphere is also higher than in an industrial atmosphere. The relative humidity in the outdoor atmosphere is consistently higher in the winter months than in the summer months. In the indoor atmosphere it is the other way round, due to the heating in winter the relative humidity is usually below 50 %.

In summary, it can be said that the problem stems from a combination of reactive surface and condensation. The condensation occurs during transportation and/or storage, whereby the conditions can change radically due to the weather. Another influence can be if the parts are not stored immediately, but remain on the truck overnight, for example, where a drop in temperature can lead to additional "moistening" of the surface.

Possible countermeasures

First of all, it should be checked whether it is possible to provide the parts with at least temporary corrosion protection. This would be the most effective method of dealing with the problem.

A second point concerns the packaging. Experience has shown that the parts are packed as they were delivered. This usually involves open crates and mesh boxes, which offer about as much protection as a revolving door as a protective fence. Nowadays, there are various types of packaging that have water-absorbent or water-repellent properties. The right material should be determined by testing the respective range of parts.

In addition, transport and storage should be optimized, whereby, for the reasons mentioned above, care must be taken to ensure that the temperature is as uniform as possible with the lowest possible humidity.

The processes therefore also explain why some deliveries in the warehouse are OK and some newer deliveries are not. OK: It is usually due to the weather (temperature and humidity) during transportation and storage, which in unfavourable cases leads to condensation/condensation on the surface, even if it still feels dry to the touch.