Learn more about the technical guidelines that drive connector development for corrosive environments to ensure a long-lasting and reliable solution in harsh conditions.
Corrosion Considerations in Connector Development
Corrosion is a process known to everyone who owns a car. In essence, rust is the corroded product of iron: Iron atoms slowly detach from the raw material when bonding to hydroxide molecules. Iron oxide is the reddish product we call rust; it has been widely used since antiquity as a pigment and is also used nowadays, for example, as a contrast agent in MRI imaging. Corrosion can affect almost any metal under specific environmental conditions. It is a chemical reaction between two materials (or two components of the same alloy) and an electrolyte. Certain materials offer great corrosion resistance, as their limit to trigger the corrosion chemical reaction is much higher than the usual chemical conditions we experience in our everyday lives.
Types of Corrosion
Corrosion is a process whereby a material degrades to a more stable state, such as an oxide or sulfide. This chemical reaction is generally triggered by external factors (exposure to corrosive agents) and can be dramatically increased by ill-adapted component design. The choice of material and that of its protective coating are critical factors to control when designing parts that will be exposed to the environment.
Three elements are needed to start a corrosion reaction – electrically conductive materials (e.g. brass, stainless steel, aluminum); components that have different electrolytic potentials (the more positive one is the cathode and the more negative one is the anode); and an electrolyte medium (e.g. salt water). A catalyst, such as oxygen or sulfur chemicals, can speed up the reaction.
Most metals (alloys) have a non-homogenous lattice, which exposes some more reactive compounds to the corrosion process.
The grain material and the intergrain material can react very differently to chemicals, and associated corrosion can vary strongly. Materials’ reactions to corrosion can differ considerably due to these structural differences (Figure 1). This inhomogeneous state affects corrosion speed, and specific corrosion mechanisms preferentially affect certain materials. Different effects on corrosion are described and categorized. It is, however, always the same chemical reaction that occurs.
The most common types include:
This very local corrosion type occurs mainly on passivized alloys. The common alloys subject to pitting corrosion are stainless steel or aluminum, since their granular composition may create local corrosion cells that generate pits.
This corrosion mechanism is generated by the difference in electrolyte potential between the grain and the inter-granular phase of the alloy. It is usually the intergranular material of the alloy that dissolves due to the galvanic corrosion effect. The material gets sandy on the surface and slowly degrades.
Crevice corrosion occurs mainly on part assemblies and is localized on parts’ interfaces. The crevice is created by a zone where, due to capillarity effects, the electrolyte accumulates, generating a mixture of electrolyte and corroded material that acts as a catalyst. The geometry of the crevice prevents the mixture from being removed, thus inducing more and more corrosion, which increases the crevice. Crevice corrosion is mainly found on bolts and plate assemblies.
Stress Corrosion Cracking
Stress is not actually a corrosion mechanism in itself but acts as a catalyst in a corrosive environment, which under unstressed situations would not corrode the material. Stress can therefore trigger a local corrosion effect.
Hydrogen is a gas that can affect the structural integrity of various materials. The mechanism is not exactly a corrosion effect, but rather an induced mechanical or chemical stress in the material lattice. Since hydrogen atoms are extremely small, they easily diffuse into the material lattice as temperature rises (thermal excitation increases gaps among atoms) and combine with material, thus creating internal stress “pockets” that can slowly shear or crack the material.
Chemical Reaction Details
Corrosion is a galvanically induced effect. The galvanic corrosion process consists in an oxide reduction chemical reaction that is triggered by the electrolytic potentials of each material present in the reaction.
In order to properly design components for use in a corrosive environment, material choice is critical. Choosing a mix of materials that have very similar electrolytic potentials is the key to success. The typical bad example is aluminum coated with chromium. The chromium offers great resistance, but once the coating gets damaged – thus exposing the raw aluminum – corrosion potential is induced. The corrosion potential can easily be seen in an electrolytic potential table (Figure 5).
Environmental parameters, such as strain temperature or microbial pollution, can make these parameters vary and, therefore, induce corrosion in commonly passivized materials.
In the following example, the electrical potential triggering the corrosion is induced by the difference in galvanic corrosion potentials and the presence of an electrolyte that allows the current to flow from one exposed surface to the other.
Aluminum has a lower potential than steel and gets corroded as it takes the place of the anode in this reaction (Figure 3). If we replaced the steel with magnesium, for example, the aluminum would no longer be corroded, as the anode would be transferred to the magnesium.
Corrosion speed is also linked to the galvanic potential intensity difference between the two alloys and the ratio between the exposed surfaces. If the anode has a small exposed surface compared to the cathode, pitting corrosion will grow rapidly and slow down as the pit surface increases, thus reducing the current flow and thereby lowering the corrosion rate.
This chemical process is commonly found on boat hulls where a sacrificial anode is used (Figure 4) – usually made of magnesium alloy, since this has one of the lowest anodic potentials in the metallic alloys class. In this example, the sacrificial anode corrodes leaving the steel hull intact – at least until the sacrificial anode has been fully corroded.
Protection Against Corrosion
These corrosion mechanisms can occur in many different situations, as materials are rarely homogeneous. Used in mechanical assemblies, they come into contact with other metals that have different potentials.
There are many mechanisms that protect parts and assemblies from corrosion; one of the most common is to coat the surfaces with a nobler material that is much less likely to get corroded. This allows the use of mechanically optimized materials while obtaining the best corrosion resistance from the coating.
The most common coating on metallic parts is nickel chromium or thin gold-based layers applied with a galvanic or sputtering deposition process.
Another solution is to add a protective phase directly into the alloy composition. This mechanism is commonly used in stainless steel. The chromium phase (16 – 18% of total mass) added to the steel reacts with oxygen (in the air, for example) to create a sealed layer of chromium oxide on the surface that protects the raw material from being exposed to the electrolyte, thus preventing corrosion (Fig. 6).
The main drawback with coating is that once the matrix material gets exposed, due to mechanical wear or pin holes, for example, corrosion speed can increase dramatically, since it is linked to the galvanic potential intensity difference between the two alloys as well as to the ratio between the exposed surfaces.
The material potential differences define the voltage, and the exposed surface ratio affects the current intensity. Therefore, if the anode has a small exposed surface compared to the cathode, corrosion will grow rapidly and eventually slow down once the surface ratio has equilibrated enough to reduce the current flow.
Common Materials and Coatings for Circular Connectors
- With nickel coating
- With chromium coating
Brass is the most corrosion-resistant material with a galvanic potential close to the coating. This ensures that, even when exposed, the current density and voltage available for corrosion is insufficient to start a corrosion reaction (in most conditions).
Nickel provides good protection but it slowly becomes corroded, and coated parts have a limited lifespan if coated only with nickel. Chromium is much more resistant to most common corrosive agents.
- With nickel coating
- With chromium coating
- Anodized (aluminum oxide Al2O3)
Aluminum is resistant in its anodized state, but this coating is highly problematic if a cleaning process is required, since the porosity of the Al2O3 structure captures microorganisms or active ions.
Aluminum is also often coated with chromium or nickel. This produces parts that offer great corrosion resistance as long as the coating stays sealed. Once the coating has been damaged through wear or scratching, the corrosion process can start in the exposed aluminum area. This failure mode is particularly dramatic as two factors contribute to the corrosion process (Figure 7):
- A high galvanic potential difference between aluminum and chromium or nickel
- The exposed surface area ratio (between the scratch and the surface of the part); the surface ratio between the exposed zone and the coating is generally high, which causes a high positive-charge density that speeds up the reaction until the anode-exposed surface area has sufficiently grown to equilibrate, thus slowing down the reaction.
As scratches are generally very small compared to the total surface area of the part, the effect is dramatically fast on these kinds of coating.
Anodizing is not a coating in itself but the artificial growth of an oxidation layer (passivation) of the aluminum surface (Al2O3).
Stainless steel is one of the best materials for corrosion resistance, since corrosion protection is embedded within the material itself. The passivation induced by the chromium phase in the alloy allows for every scratch or wear to re-coat automatically, thus preventing corrosion from appearing (Figure 6).
There are several grades of stainless steel with various compositions, making it optimized for parts machining (AISI 304) or welding, for example.
Stainless steel offers great chemical resistance and an outstanding level of passivation layer protection against standard environmental corrosion. This makes it very difficult for active ions or microbiological entities to diffuse into the material lattice, ensuring that equipment is very easy to clean and sterilize.
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Author Jacques Miéville is a project manager at Fischer Connectors.