Proper Care and Feeding of Materials in the Brewery
Fred Holzhauer, B.S. Met. Engr.
Materials Science – Structure is the Key to Performance
Metals and their alloys define their performance by the grain structure achieved from the work of manufacture and handling, as well as the composition of the grains and intergranular material evident in their final form. The discipline of Physical Metallurgy addresses alloys and their properties. Metal is formable and tough, but heavy, and inherently victim to corrosion. Corrosion can start granularly or intergranularly. Many “tricks” have been developed to inhibit metal’s tendency to corrode. Metals clean well, and have only slight flavor profile.
Glass is amorphous, which means it has little long range structure. It more resembles a supercooled liquid in structure than most other solid .Debate over these statements still rages on in the scientific community, but here, you are subject to my opinni. Glass can be considered silicate “soup.”
Glass is very inert, chemically, subject only to Carbonic Acid and Fluoride corrosion in aqueous (water based) systems. Please note that Carbon Dioxide exists in beer as Carbonic Acid.lass cleans well, and has no flavor profile. Glass is subject to difficulty with manufacturing as well as being inflexible and fragile to impact or tensile stresses.
Refractory Silicates (Tile, Grout, Concrete)
Mostly reserved for floors, walls, and décor, these materials resemble glass in having silicates in common, as well as the friability property. They are fairly water resistant, but some of the alkaline oxides, that are present in the composition along with the silicates, provide weakness to chemical attack, especially acids. Refractory Silicates are highly stable, thermally.
Plastics are inexpensive, durable, light, workable, and resistant to hydrolytic decomposition.They usually contain additives that impart objectionable flavor profiles to beer. Once scarred, they become extremely difficult to clean properly. They are generally not suited to heated processes. Very little plastic is observed around brewing, but its use is bound to increase.
Corrosion Aspects of Common Brewery Materials
Aluminum—Aluminum is amphoteric, meaning it corrodes both from acid or alkali attack .Highly susceptible to Chlorine Bleach. Aloy elements affect performance and corrosion properties. Copper and Silicon are common alloy elements. The copper can form conversion coatings (described below).Not very prevalent around the brewery.
Brass—Don’t use brass for anything except natural gas lines. Extremely vulnerable.
Copper—Soluble in just about anything. Easily improved by conversion coating. For example, just the hardness in your water will form a very resistant film on a copper surface.
Steel—Subject to oxidation and acid attack. Salt is a strong intermediate to corrosion. Can be improved with conversion coating.
Iron—Silicon, Carbon and intragranular Carbides that are present beyond the specifications for steel tend to help to resist hydrolytic (acidic or basic) attack. Will oxidize readily.
Stainless Steels—Passivates; Chromium in the alloy oxidizes to a refractory (oxidatively inert) material. Halogens can replace Oxygen, undoing this protective effect, therefore Hydrochloric or Hydroflouride acids can still cause trouble.
Refractory Silicates (Tile, Grout, Concrete)
These are Alkali Metal Silicate Oxides. Not Prone to Oxidation.Aain, Halogens can replace Oxygen in their structures; most alkali – halogen compounds are very water soluble (e.g. table salt). Dilute acid is an effective cleaner, if not overused to the point of corrosive damage.
Epoxies are reacted with amines to form crosslinked plastics in situ.Newer chemical resistant finishes are good against most solvents, acids, and bases. Residual amine is very prone to attack by halogens, for instance, Cl from Hydrochloric Acid or Chlorine Bleach.
These natural or artificial gums are cured with sulfur compounds. Sulfur compounds suffer from Oxygen attack, becoming Slates like Nitric Acid or Chlorine Bleach will harm rubber on extended exposure.
Almost chemically inert, these substances don’t react readily, but they can be dissolved by strong or weak polyprotic (having more than one Hydrogen on the molecule) acids on extended exposure. They are difficult to remove or attack without very good surfactancy and an appropriate strong solvent, like methylene chloride or acetone.
Metallurgical Philosophy – Be Aggressive About Passivation
There seems to be some confusion about the difference between passivation effects and conversion coatings amongst those writing to the brewing industry magazines.
Passivation in the brewery is accomplished only on stainless steel.By oxidizing Chromium in the alloy to Chrome Oxide, typically with Nitric Acid, we create a surface barrier that resists oxidative attack.The normal action of stainless steel is to react to rust-forming conditions with the formation of the refractory Chrome Oxides. In the case of organic acid attack, such as that experienced with beers, we don’t have a pure rusting condition. The attack is by a process called chelation which can slowly subvert the stainless steel’s passivated surface. To counter this effect, we need to force the formation of the refractory surface with the passivation process.
In the act of acid corrosion, both solubilization and oxidation take place. It is only the oxidation portion of acid metal washing that can accomplish passivation. Use an oxidizing acid, such as Nitric Acid, to get an adequate passivation. Choose a nitric / phosphoric acid formulation for routine maintenance, so your steel passivation will stay strong.
Conversion coatings are unlike passivation in that they rely on a multiple mechanism to form, and they require material from both the nearby solution and the metal itself.The metal substrate first must be leached (slightly corroded) to make metal ions available for reaction. Then the metal ions must be oxidized to the proper state. Finally, the metal ions must react with the nearby solution to form a precipitate that limits further corrosion.
This process almost never happens by accident. It takes a careful chemical design to accomplish this, especially in a cleaner formula. Copper is an exception. It naturally converts with some water hardness ions.
Nearly all automobiles, metal street signs, and metal toolboxes undergo commercial conversion coating processes prior to painting.
304 or 316 SS, Which One to Use?
Grain structure is the principal thing that affects a metal’s physical properties. Hardness, malleability, and corrosion resistance can all be related to the combination of chemical makeup and grain structure. Metallurgists spend a lot of effort trying to create grain structures by alloying, heat treating, and working a metal to achieve desired properties.
Both 304 series and 316 series stainless steels have “austenitic” grain structures, with similar physical strengths. The chief difference is that 316 contains 2% more Nickel and has some Molybdenum. 316 is more costly to manufacture, and machine or weld.The 316 resists attack better in salty (marine) environments. Both types should be low carbon grade for welding, unless a further heat treatment is specified. Generally, 316 is more appropriate for the brewery.
There are other grades of stainless, to be sure, but you might be faced with this particular choice for a replacement component.Go ahead and pony up for the 316.
Corrosion – Good, Bad, and Ugly
This is GOOD corrosion. Used to clean and brighten metals that have beerstone buildups, or perhaps oxide buildups such as on the outside of a copper kettle.
BAD corrosion. This is caused by precipitation of uncontrolled water hardness.It may occur in any aspect of brewing or cleaning. Often, but not always, the precipitation is motivated by a corrosion cell (think about two halves of a battery) ongoing at the surface of some equipment.
The UGLY corrosion. It’s gone on too far.It needs remedial action to stop it. It harbors micro-organisms. Near to impossible to properly clean.
Proper Care and Feeding
The enclosed chart shows corrosion resistance of common materials to some of the popular chemical types in the marketplace.
Use a Nitric-Phosphoric blend general purpose acid for removing beerstone and oxides. You benefit from the solubilizing power of Nitric Acid, and you can accomplish passivation and conversion coating with a single product. Operate near a pH of 2. Avoid pH range 3.5 to 5.0.
Never use Sulfuric or Hydrochloric Acids for anything.
If you choose a caustic-chlorine approach, don’t allow it to dry on a surface, even Stainless Steel. You will create irreversible etching that will lead to the UGLY corrosion.
Recycling cleaners is bad practice, even though economical. Not only do you recycle soil, but every time you make up a chlorine level, you create more potential carcinogens, both in your brewery, and downstream. Also, conductivity probes tend to give false highs as they read ionic soils as well as clean. Choose Oxygen chemistry instead of Chlorine, when you can.
Newer non-caustic chemistry relies on Oxygen donors. They have nicer environmental profiles, both upstream as well as downstream. It takes a little finesse to use, but is worth learning about.
Treat Nitric Acid, Hydrochloric Acid, and Chlorine Bleach like carbonated beverage. Too hot drives off soluble gasses. Hotter is not always better.130 °F, maximum.
Don’t play chemist. If it’s not working at a recommended temperature or concentration, ASK.
Never mix chemicals without explicit instruction to do so.
Substrate Resistance Chart—10% Aqueous Solutions Above 100 °F
|Iron (High Si)||U||E||G||U||E||G||U|
U = Unsatisfactory
R = Resistant
R* = corrosion resistant, but embrittles with time
S = Satisfactory
G = Good
E = Excellent
The combination of nitric and phosphoric acids forces conversion coatings on aluminum, copper, and steel.