Confronting corrosion

Jack Tillotson discusses stopping the corrosive effects of poorly maintained tablet tooling

In today’s fast-paced pharmaceutical and nutraceutical industries, manufacturers face the challenge of producing tablets quickly and efficiently to meet the growing demand while overcoming challenging production issues that arise daily when producing solid dose forms.

One area that is hugely influential to the success or failure of mass-produced tablets is the condition of the tooling. The availability and condition of punches and dies are important for uninterrupted production. If tooling is not in optimum condition, problems such as sticking, contamination issues or corrosion are commonplace.

By implementing a planned professional maintenance and management process, tablet tooling can help to boost production, prevent the rejection of tablets, reduce formulation waste, minimise press damage, avoid press downtime and increase profitability.

How does corrosion form?

Corrosion of tooling is one of the biggest production problems that can occur in tableting due to poorly maintained tablet tooling and corrosive formulations that may contain acids, salts or Chlorine for example. Corrosion will be identified by the appearance of discolouration, etching or red rust on the punch and is most often found on the punch tip face, as this is the section of punch that is in contact with the formulation for the longest period of time.

Acidic substances found in formulations are often the main cause of the corrosion. It can also occur when iron particles in the steel tooling are exposed to oxygen and moisture in the form of humidity or vapour.  When the steel is exposed to water, the iron particles are lost to the water’s acidic electrolytes. This means they oxidise the iron particles forming areas of corrosion on the punches and dies.

Excess humidity in the compression room, tool storage area or places where the formulation is stored before use, can have a significant impact on corrosion developing on the tooling. It is, therefore, extremely important to regulate the temperature and humidity and minimise any moisture to prevent corrosion of the punches and dies.

Types of corrosion

Several types of corrosion can form on tooling. One of the most destructive is ‘pitting’. This is typically localised and can penetrate much deeper into the material’s surface than the visible hole on the exterior would indicate. This results in weakening the integrity and strength of the tooling and can be caused by non-uniformities in the steel structure.

The most common type of corrosion is ‘uniform’. As the word suggests, it evenly attacks across the surface of the material. It is, however, the most benign corrosion affecting the mirror finish surface of the punch tip face. However, this type of corrosion can turn into pitting corrosion if not addressed in a timely manner. It typically occurs over reasonably large areas of the surface, reducing the strength of the tooling. Just like pitting, uniform corrosion can be caused by non-uniformities in the steel structure.

A further example of corrosion is ‘galvanic’. This type of corrosion is induced when two dissimilar metals are coupled together within a conductive solution. The difference between the metals is determined by its notability and therefore it’s corrosion potential. Galvanic corrosion is a local form of corrosion, and it is limited to the contact zone between the two metals. The intensity of corrosion decreases by increasing the distance between the point of contact.

How to stop the rust

Prevention is better than cure, and in this instance that statement is very true. Since it’s impossible to eliminate oxygen from the air, it is necessary to implement measures to regulate temperature and humidity and minimise moisture.

Effective cleaning measures are another effective step in corrosion prevention. Inadequate and incorrect cleaning methods can lead to corrosion. Certain formulations that contain corrosive elements can react with the tooling surfaces and cause oxidation if not cleaned properly. Therefore, it is vital that tooling undergoes a thorough cleaning process after compression, preferably by use of ultrasonic, and always using a corrosion inhibitor.

After cleaning and drying lubrication is another important maintenance step to consider. Tablet tooling should always be lubricated before storing to protect it from moisture and prevent corrosion. It is also important to note that drying is very important before lubrication. If moisture sits between the lubricant and the steel substrate, then filiform corrosion can occur.

Two of the most effective lubricants are oil and grease. There are various grades of each type of lubricant. Measured in viscosity, these grades characterise the ease at which the lubricants will flow. Oil is recommended when storing tooling for a short period of time. They are available as synthetic vegetable or mineral oil and are the industry’s most common forms of lubrication.

For long-term tool storage, grease is the optimum choice as it is more viscous than oil adhering to the tool for a longer period of time. This offers greater protection for prolonged storage, however, it is worth noting that it is harder to remove from tooling than oil.

Although using the correct maintenance and storage processes are vital to stop corrosion, it is important to note that high-quality tooling steel (normally a martensitic stainless with a high chromium content) and coating technology can have a considerable effect. Consider using specialised steels and coatings with corrosion resistance properties if a product that you are compressing has corrosive elements to its formulation. Coatings in particular have a very high resistance to oxidisation, staining and discolouration of the tablet tooling.

Address the corrosion cause

The consequences of corrosion can be costly, not just in purchasing additional tools, but also in production problems that could have been avoided. Poor tool care, the environment, storage, and a lack of understanding of the formulation’s characteristics will all contribute to corrosion.

Selecting the right tool steel and coating is key to the successful performance of tablet compression tooling and the prevention of corrosion.

It is important to consult with a tooling expert who can determine which steel and coating applications are the right fit the product and provide optimised maintenance solutions for your specific requirements.

Jack Tillotson is process development engineer at I Holland

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