Thursday 31 March 2022

How to passivate stainless steel homebrewing equipment

We recently purchased a new stainless steel fermenter. Cheeky Peak Brewery, the manufacturer, recommend passivation of their equipment before first use, and even offer a guide with a few different methods/options on how to accomplish this.

First things first though - what exactly is passivation and why do we need to do it? Well, stainless steel is made from iron, carbon, chromium and other elements - and the chromium content creates a thin layer, known as a passivation layer which protects and prevents oxygen from accessing the iron content of the stainless steel. If oxygen is able to access the iron content - rust deposits will form.

The process of manufacturing, drilling, welding, or using harsh/abrasive cleaners and cleaning agents can break down this passivation layer so it's recommended practice to passivate new equipment before first use - as a type of insurance policy against any issues with the passivation layer. Better to be safe than sorry, right?

The first step with passivating is to thoroughly clean the surface of any residual oils and dirts - powdered brewery wash (PBW) is sufficient for this in most cases - preferably done with warm/hot water. Pretty straight forward and standard for most homebrewers.

Next, we need to treat the surface with an acid. The acid chemically removes any free iron from the stainless surface, which then leaves behind a uniform surface with a higher proportion of chromium than the underlying stainless surface. Upon subsequent exposure to oxygen in the air after the acid treatment, the stainless steel will form a chromic oxide layer over the following 24 to 48 hours.

There are typically four (4) types of acids commonly used to accomplish this;

1. Nitric acid
2. Oxalic acid
2. Citric acid
4. Phosphoric acid

And this is where it gets complicated. Firstly, nitric acid is very corrosive - making it dangerous to use, and is also harmful to the environment, so we're going to opt to not use this one.

Oxalic acid is a strong organic acid that naturally occurs in many known fruits and vegetables like rhubarb, spinach - making it a good candidate to use for passivating. It can be found in a common and popular cleaner called Bar Keepers Friend - and Short Circuited Brewers on YouTube made a video on how to passivate stainless equipment by making a thick paste with Bar Keepers Friend powder. The problem for us though is that in Australia, Bar Keepers Friend is made with citric acid, not oxalic acid like it is in the United States. One important thing to note though is that oxalic acid may remove or fade etched markings on your stainless equipment, so be careful and mindful of this if you decide to use an oxalic acid based solution.

Perhaps this isn't really a problem though, since citric acid is a recommended acid to use for passivating. SS BrewTech, a manufacturer of stainless brewing equipment actually recommend citric acid for passivation since it's environmentally friendly and readily available in most parts of the world. The previously mentioned guide from Cheeky Peak recommend a solution of 4-10% citric acid by weight with clean water for a citric acid passivating solution. Australia's version of Bar Keepers Friend has 5-10% citric acid in it by weight, so perhaps we could use the same process as Short Circuited Brewers YouTube video and make a thick paste by mixing Bar Keepers Friend powder with a little bit of water (they used a ratio of 2:1 BKF to water).

On top of this, NASA have performed a study and comparison on the effectiveness of citric acid and nitric acid when passivating stainelss steel. Looking at the photo comparisons in their report, it's easy to see that citric acid was more effective at preventing rust from occurring when compared to the use of nitric acid.

Lastly, we have the option of phosphoric acid. This is most commonly found in homebrew sanitisers like Star San. We have an equivalent product from KegLand called Stellar San. Numerous sources recommend passivating with a Star San solution five times the recommended strength for general sanitising. The recommend strength is typically 1.5ml/L, so for a 30L vessel this would equate to 225ml of Star San. It's not particularly expensive, but that's still quite a lot to use in one go. And given it's a significantly stronger mix than recommended, it can't be re-used for general sanitising of equipment as the increased strength would potentially lead to off flavours - so we're not really liking this option.

KegLand have a blog post stating that passivation on stainless equipment can be done by simply wiping undiluted Star/Stellar San onto the stainless surface, leaving it for 24 hours then wiping away. However, there are other sources stating that phosphoric acid is not a suitable solution for passivating - like the Australian Stainless Steel Development Association who state that;

"Phosphoric acid will remove rust and sulphide inclusions, but it is not oxidising and will not strengthen the passive film"

There are plenty of other sources stating similar things, so we're thinking creating a paste with Bar Keepers Friend may be the way to go - which would essentially mean we're using a citric acid based solution. 

The wealth of information on the internet on any subject can be a blessing and a curse. A quick google search on methods of passivation led us down this rabbit hole, and we were quickly overwhelmed by the vast amount of (often conflicting) information, hence why we've made this post to help summarise our findings - and also provide sources for our information.

What method do you used for passivating your stainless equipment? Let us know in the comments below.

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