How Pickling and Passivation Work

We recently explained how you can prevent corrosion in your headworks. We also let you know about how we treat the stainless steel we use to create the headworks equipment for wastewater treatment facilities around the world.


Now we’d like to go a little deeper into our process for protecting the equipment that will go into your facility. In particular, we’ll look at the steps we take for pickling and passivating stainless steel during manufacturing.

Why pickling and passivation matter

First, let’s briefly recap the value of pickling and passivation. Pickling the steel removes its impurities. It also limits the number of potential corrosion points. At Hydro-Dyne’s manufacturing facility, we use a total immersion process that is highly effective and ensures 100% of the parts are covered by the acid mixture. More on that below.


Passivation is another important step in preparing the steel for many years of service in your wastewater treatment facility and the corrosive elements within.  Passivation is the process of adding a chromium oxide layer that protects the surface. At Hydro-Dyne, we use nitric acid to passivate the steel, taking care not to damage the surface.

Four-step method of pickling

Hydro-Dyne follows industry standards for treating stainless steel parts. To do this, we use a four-tank method of pickling. Each tank has its own purpose, as seen in the following:


  • Tank 1: Steel is pre-cleaned
  • Tank 2: Cleaning residue is rinsed
  • Tank 3: Every item is descaled and passivated using nitric acid and hydrofluoric acid
  • Tank 4: Residual acid is removed during the rinsing process.

We rinse the items for 25 minutes in deionized water that is pH balanced. Because we manufacture the materials to be fully drainable, we are able to completely remove the acid and water from the steel. In this process, all the surface area is passivated.

To ensure quality during passivation

The passivation stage is critical because this is where we form the protective chromium oxide layer. During the passivation step, we take the following steps to ensure quality:


  • We monitor every tank
  • We keep the iron content below a certain level
  • We adjust the acid content per our quality program to keep an adequate percentage
  • We recycle water where we can

We follow ASTM standards

Hydro-Dyne follows ASTM standards during this treatment process. The A380 standard covers the best practices for cleaning, descaling, and passivating stainless steel. The A967 standard is the specification that addresses the passivation treatments for stainless steel.


Hydro-Dyne has taken these standards and modified them for our ISO 9001 process. ISO 9001 is a quality program that we set up set up for our design and production processes, which include:


  • Material segregation (so that the equipment used matches the intention of the design)
  • Pickling/passivation
  • Assembly controls that ensure the right materials and tools are used to put together the screenings and screening handling equipment
  • Unit testing
  • Tank hydrotesting

For more information about how we go about designing our headworks and screening equipment, take a look at our blog post “How to Prevent Corrosion in Your Headworks.”

Work with Hydro-Dyne to manufacture the best headworks available

We take the design, treatment, and assembly of every piece of equipment very seriously, and we hold ourselves to the highest standards. If you want to make sure your wastewater treatment plant has the best materials in its headworks, work with Hydro-Dyne. We are here to answer in detail any questions you have. We also highly recommend you come to our facility to see our process in person. Contact Hydro-Dyne and we can arrange a visit to our Clearwater facility.


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interest in our company.

Do you have any questions or would you like more information? We’d be
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