Critical literature review about the affects of using chromic acid Vs tartaric acid when anodising Aluminium for industrial use.

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Bioscience & Chemistry Programme
Faculty of Health and Wellbeing
Professional & Scientific Practice 3
Comparison of Chromic Acid and Tartaric Sulphuric Acid in the process
of Anodising Aluminium
By Molly Winterbottom
Submitted: 29/09/2023
Supervisor(s): Daniel Allwood (APA)

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CONTENTS:
Abstract - 3-4
1.0 Understanding the process of Anodising - 4
1.1 What is Anodising? - 4
1.2 How does it work? - 4-5
1.3 What can it be used for? - 5
2.0 Chemicals used for Anodising.
2.1 Chromic Acid - 5
2.1.1 What is Chromic Acid? - 5
2.1.2 The role of Chromic Acid in the Anodising process - 5-6
2.1.3 Health and Safety around Chromic Acid - 6-7
2.1.4 Benefits to using Chromic Acid in the Anodising process - 7
2.1.5 Cons to using Chromic Acid in the Anodising process – 7-8
2.2 Tartaric Sulphuric Acid - 8
2.2.1 What is Tartaric Sulphuric Acid - 8
2.2.2 The role of Tartaric Sulphuric Acid in the Anodising process –
8-9
2.2.3 Health and Safety around Tartaric Sulphuric Acid - 9

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2.2.4 Benefits to using Tartaric Sulphuric Acid in the Anodising
process. - 9
2.2.5 Cons to using Tartaric Sulphuric Acid in the Anodising
process. – 9-10
3.0 Comparison of Chromic Acid and Tartaric Sulphuric Acid - 10
3.1 Effects on production – 10-11
3.1.1 How the structures change over time - 11
3.2 Health and Safety risks - 11
4.0 Conclusion – 11-12
Abstract
This paper aims to identify two possible ways of carrying out the anodising process on
aluminium alloys. These alloys are widely used in industries such as car and aviation
manufacturing as the additional increase of resistance to corrosion benefit the seller and
buyer of the product. Due to an incline in the interest of health and safety of production
workers, a well-known chemical used in the anodising process has started to be sieved
out of companies and their factories. Chromic acid has been widely used within the
industry for its beneficial interaction it has with aluminium alloys along with other
metals. However, its risk has been identified as much greater than its beneficial use and
a new alternative was suggested. Tartaric sulphuric acid or TSA was introduced as this
alternative and has been taken on by many industries that previously used chromic acid.

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In this paper the main differences between the two acids will be discussed along with
their pros and cons when being used for the anodising process, most importantly
mentioning their health and safety risk factors.
1.0 UNDERSTANDING THE PROCESS OF ANODISING
One of the main issues to either prevent or solve when dealing with metals like
aluminium is their tendency to corrode. The use of Anodising helps to prevent corrosion
by increasing the metals corrosion resistance.
1.1 WHAT IS ANODISING?
The process of Anodising can be used on machined aluminium parts in order to protect
them from corroding. This process is widely used in industries such as aviation and motor
manufacturing.1
The protection from corrosion comes from the inorganic coating layers
that can be incorporated onto the bare metal after the organic oxide layer has been
removed.2
1.2 HOW DOES IT WORK?
The process of anodising works by creating a porous oxide film on the bare metal which
resemble hexagons in their shape (Figure 1) but can vary with their distance and size

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depending on the voltage and electrolyte used, along with the time the anodising
process occurs for.3
Figure 1. Hexagonal cylindrical pores
1.3 WHAT CAN IT BE USED FOR?
There are multiple types of anodising, including chromic, sulphuric, and hard anodising.
All 3 are used across industries in the UK for treating metals when manufacturing motor
vehicles, aircraft along with other companies looking to produce metal components that
are resistant to corrosion in order to prolong its life. Along with corrosion resistance,
electrical insulation and abrasion resistance can be instilled into the aluminium panel
from the anodising process.
2.0 CHEMICALS USED FOR ANODISING
2.1 CHROMIC ACID
2.1.1 WHAT IS CHROMIC ACID?
Chromic acid (H2CrO4) is a strong oxidising agent4
formed from the reaction of chromium
trioxide with water5
. Along with its hazardous nature (See section XXX), it is unstable.
Due to its stability, it must be created in situ6
.

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Figure 2. Molecular structure of Chromic Acid
2.1.2 THE ROLE OF CHROMIC ACID IN THE ANODISING PROCESS
The process of anodising speeds up the production of the aluminium’s natural oxide
layer, in order to do this with chromic acid, the aluminium needs to be submerged in a
chromic acid electrolytic solution which would then have a certain amount of DC
electrical current being generated through it, which is connected to an anode of an
electrical circuit, along with another metal electrode (cathode) which is submerged in
the chromic acid bath. Thus, creating a coating on the aluminium that is flexible, thin,
dense, and dark grey in colour.
The chromic acid allows the aluminium to extract negative oxygen ions from the chromic
acid bath as the positive ions from the aluminium as drawn to the cathode leaving the
aluminium to become porous due to the lack of positive ions, giving way for the negative
ions to react forming the oxide layer.7
2.1.3 HEALTH AND SAFETY AROUND CHROMIC ACID (TOXICOLOGY)
When dealing with Chromic acid, many precautions need to be considered, as inhaling
chromic dust can cause severe irritation to the lungs, nose, and bronchial tubes. Short
term exposure to chromic acid produces severe side effects some may consider being
associated with long term exposure effects to other chemicals, putting it into perspective
how dangerous it can be dealing with chromic acid.
Short term exposure effects:

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• Severe irritation to lungs, nose, throat and bronchial tubes
• Severe injury if splashed in the eyes
• If swallowed, may cause kidney and stomach problems
• Skin exposure may cause the skin to ulcerate
Long term exposure effects:
• Perforation and ulceration of nasal septum
• Onset asthma effects
• Liver damage
• Lung cancer8
2.1.4 BENEFITS TO USING CHROMIC ACID IN THE ANODISING PROCESS
As the process of anodising is used worldwide for the main purpose of creating
resistance to corrosion on metallic parts it is generally cheap to use. Many benefits of
using chromic acid for the cost-efficient anodising process including its corrosion
resistance properties along with an enhanced capability of bonding adhesives due to it
providing a surface with sufficient roughness enabling glue to stick to it along with paint.9
2.1.5 CONS TO USING CHROMIC ACID IN THE ANODISING PROCESS
When looking at section 2.1.3 on the Health and Safety around Chromic acid it is obvious
that one of its most significant downfalls of using it for the anodising process is its
detrimental affect on the workers dealing with the chemical. According to a paper
written on the environmental impact of using chromic acid it also effects plants as well

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as humans. The paper states that its use can create toxicity in plants which therefore
impact the roles and purposes a plant has including decrease in growth and seed
germination along with interferences with its need to photosynthesise.10
2.2 TARTARIC SULPHURIC ACID
2.2.1 WHAT IS TARTARIC SULPHURIC ACID?
Tartaric Sulphuric Acid or TSA is made from the mixture of tartaric acid and sulphuric acid in
anodising baths and the required concentration needed to anodize the material being
manufactured.
Figure 3. Molecular structure of Tartaric Acid
Tartaric Acid is a naturally occurring acid found in most fruits. It can take on l(+), d(-) and dl
racemic stereoisomeric forms, the most common form used when anodising metallic
materials is the l(+) isomer11
2.2.2 THE ROLE OF TARTARIC SULPHURIC ACID IN THE ANODISING PROCESS
Tartaric acid and Sulphuric acid are mixed in a specific ratio to form the electrolyte solution.
Once placed in a suitable UV bath the aluminium is submerged and an electrical current is
passed through the bath. The hydroxide, oxide and sulphate ions are pulled in the direction
of the anode whilst the positive aluminium ions are pulled towards the cathode and Al3+
ions

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interact at the anode surface with the oxides forming the synthetic oxide layer on the
aluminium alloy.
2.2.3 HEALTH AND SAFETY AROUND TARTARIC SULPHURIC ACID (TOXICOLOGY)
Although TSA has been suggested and used an alternative to chromic acid due its chromic-
free nature, this is not entirely true. The Tartaric sulphuric acid anodising post-process may
use the trivalent chromium (Cr(III)) in the sealing process. Fortunately, this variant of
chromium is non-carcinogen unlike the hexavalent chromium found in Chromic acid
anodising.
Although still containing toxicological risks and hazards such as dermal, inhalation risks and
ocular effects, TSA can be considered significantly safer than chromic acid.12
2.2.4 BENEFITS TO USING TARTARIC SULPHURIC ACID IN THE ANODISING PROCESS
Similarly, to the benefit of using chromic acid, the use of tartaric sulphuric acid for anodising
aluminium can also increase the metals resistance to corrosion. Another benefit found when
using TSA appears on the finished product as it produces an aesthetic finish on the
aluminium parts.
2.2.5 CONS TO USING TARTARIC SULPHURIC ACID IN THE ANODISING PROCESS
When working with aluminium parts, especially in the aviation industry, may tests are done
on test pieces after they have gone through the surface treatment facility; One of these tests
is fatigue testing. It’s been found that when anodising using tartaric sulphuric acid (TSA)
there is a reduction in the fatigue limit of the aluminium, which as a result, can lead to

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premature failure of the part if this reduction in fatigue strength is not considered when
designing the part.13
Another downfall to using tartaric sulphuric acid (TSA) over chromic acid is the extra care
that needs to be considered for the baths of the surface treatment facility. The use of UV
lights is a necessity due to the growth of microorganisms in the bath as it acts as a feeding
source for the bacteria. In order to solve this a quick but expensive solution is needed, in
many cases getting rid of the bath and refilling it with fresh tartaric sulphuric acid solution.
However, this is costly to production in many companies and even though deemed the
quickest solution, can take a few days to be carried out depending on the size of the surface
treatment bath.14
3.0 COMPARISON OF CHROMIC ACID AND TARTARIC SULPHURIC ACID
3.1 EFFECTS ON PRODUCTION
It has been found that when using tartaric acid in a sulphuric acid anodising bath, the tartaric
acid slows down the rate at which the porous making up the anodic film are grown at. The
cause of this from one study is said to believe it is due to the reduction in the dissolution
rate of the naturally occurring Aluminium oxide layer when growing the new synthetic
anodic film layer.15
Chromic acid was the original electrolyte used for the process of anodising, although due to
it having a hexavalent nature, making it a very strong oxidising agent, its use has been
restricted by law and regulations being put into place as it requires a lot of precaution and

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care when handling therefore alternatives like Tartaric sulphuric acid (TSA) have been
suggested to use to prevent any incidents with workers and their lack or insufficient PPE.16
3.1.1 HOW THE STRUCTURES CHANGE OVER TIME
There are a number of ways to test aluminium once treated, one of these things is
performing fatigue testing. By taking a test piece and placing it through the same process the
aluminium would be chemically treated in the surface treatment facility and in the heat
treatment, a representative test piece can be used to test the fatigue of that aluminium
panel. It was found in studies using both chromic and tartaric sulphuric acid that the fatigue
level was increased, and the test piece could tolerate more stress. Although when comparing
both acids, it was found that using tartaric sulphuric acid gave a slightly decreased fatigue
level.17
3.2 HEALTH AND SAFETY RISKS
Whilst this paper has already discussed the health and safety of each chemical, when
comparing both side by side, evidence from many studies have concluded chromic acid to be
the most dangerous of the two. Due to its severity the European Chemicals Agency banned
the use of any hexavalent chromium in surface treatment facilities in 2017.18
4.0 CONCLUSION
To summarise, the various studies done on the effect of chromic acid verses TSA on
anodising, come to a clear conclusion ruling TSA over Chromic acid on a health and safety
aspect but may side with chromic acid on the bases of reproducibility of anodised aluminium

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parts, with less issues around tank maintenance and possible rework on already anodised
parts.
References:
1 Sina Ebnesajjad, Handbook of Adhesives and Surface Preparation, Technology, Application and Manufacturing,
2011, Pages 83-106
2 M Ardelean, S Lascau, E Ardelean, and A Josan, Materials Science and Engineering, Surface treatments for
aluminium alloy, 2018, Volume 294, pages 1-12
3 E M Aryslonova, A V Alfimov, S A Chivilikhin, Journal of Physics, Modelling the Growth Process of Porous
Aluminium Oxide Film during Anodization, 2015, Volume 643, Pages 1-5
4 Arthur P. Sanford, Total Burn Care, Chemical Burns, 2007, Volume 3, Pages 536-541
5 Arun Kumar Sharma DSc, FNA, FASc, Archana Sharma DPhil, DSc, FNA, FASc, Chromosome Techniques,
Fixation, 1980, Volume 3, Pages 30-70
6 https://chem.libretexts.org/Ancillary_Materials/Reference/Organic_Chemistry_Glossary/Chromic_Acid
(Accessed 14/08/23)
7 https://www.xometry.com/resources/machining/chromic-acid-
anodising/#:~:text=Chromic%20acid%20anodising%20is%20an%20electrochemical%20technique%20that%20u
ses%20a,hard%20coating%20of%20aluminum%20oxide. (Accessed 11/09/23)
8 Mancuso, T.F, Occupational Cancer and Other Health Hazards in a Chromate Plant: A Medical Appraisal II.
Clinical and Toxicologic Aspects, Industrial Medicine and Surgery, 1951, Volume 20, pages 393-407
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10 Helena Oliveria, Chromium as an Enviromental Pollutant: Insights on Induced Plant Toxicity, Journal of
Botany, Vol.2012, pages 1-8
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12 Desmond I.Bannon, PhD, DABT and Valerie H.Adams, PhD, DABT, Toxicology Report No. s.0052729.8-16,
Toxicology Assessment for Safer alternatives for Readiness (SAFR) Work Unit TMR 15-01 Chromium Free
coating Tartaric Sulfuric Acid Anodising (TSAA), 2016-2019, Pages 1-50
13 https://www.xometry.com/resources/machining/sulfuric-acid-anodising/ (Accessed 13/09/23)
14 Alessandro Di Cerbo, Andrea Mescola, Romona Iseppi, Roberto Canton, Giacomo Rossi, Roberta Stocchi,
Anna Rita Loschi, Andrea Alessandrini, Stefano Rea, and Carla Sabia, Antibacterial Effect of Aluminium Surface
Untreated and Treated with a Special Anodising Based on Titanium Oxide Apporved for Food Contact, PubMed
Central, 2020, Vol 12, Pages 450-456
15 M. Curioni, P. Skeldon, E. Koroleva, G. E. Thompson, J. Ferguson, Journal of The Electrochemical Society, Role
of Tartaric Acid on the Anodising and Corrosion Behavior of AA 2024 T3 Aluminium Alloy, 2009, Volume 4,
Pages 147-153
16 https://metalfinishingsltd.co.uk/treatments/chromic-anodising/ (Accessed 16/08/23)
17 http://etd.lib.metu.edu.tr/upload/12623003/index.pdf (Accessed 15/09/23)
18 https://surfacetreatment.johncockerill.com/news/as-of-2017-hexavalent-chromium-can-no-longer-be-used-
in-aluminium-surface-
treatment/#:~:text=The%20REACH%20Regulation%20of%20the,surface%20treatment%20as%20of%202017.
(Accessed 18/09/23)